A few thoughts: (numbered by major suggestion)
1) ever see "The Chipmunk Adventure" (animated)? 2 teams taking different routes on a round the world balloon race, neither knowing they they are really smuggling diamonds...
2) Zelda syndrome (or just basic computer game and Adventure Path). The crystals, triforce fragments, ect... are guarded by progressively more difficult bosses with progressively tougher minions in progressively more complicated dungeons in progressively more distant locations....
do you want to avoid that?
3)You could have a higher starting level and make the dungeons all roughly equal in difficulty. However, as the pcs begin to collect the orbs, additional (even non thematic for that dungeon) guardians are dispatched to protect the remaining orbs, and the pcs begin to be attacked and harassed by others wanting to either return the orbs or steal them for their own purposes. Sometimes the areas near the dungeon can be more difficult than the actual dungeon.
4)OR, you can start at low level, make the first orbs be "Lost" ones not being "properly guarded" being used as a watch fob by an ogre/gnome or part of royal scepters or something of the like. (evil party hired to do a "Shadow run" to steal the crown jewels of a minor kingdom maybe....)
5) The real adventure starts when the pcs find out what they have acquired.... and the wards on them.
Perhaps
A: The orbs cannot be Identified, scryed upon or detected except at point blank range, and hedge out srying for a few miles (and maybe that was why it ended up in the royal scepter. It was mis IDed as a relatively mid-powered artifact "security device"). After the pc's start carrying them around, however, their opposition can start "going to the middle of the zone we can't scry on" to find them.
B: They cannot be teleported or placed into any extra or non dimensional space. They are tied to the material plane. Perhaps they also block teleports, summonings and etherial/astral/elemental(earth glide/passwall/windwalk) travel within 400 ft (and are not always being so much "guarded" as "being used to plug this hell gate" or "is keeping the vampire/banshee/whatever trapped in the bricked up room behind the pedestal"{the good party might have to leave the orb to protect the city, and go get another item/ritual/spell caster to plug the hole, and then have to fight off the extra- dimensional invaders when they come back as the evil party stole it} )
C: They are warded by a GODS level deviation spell. If any being who a)knows {sorta}what a Dragon Orb is, and b)knows the approximate location of one, and c) has any, even the slightest, intention of trying to obtain one.... attempts any long range transportation spell (maybe even wind walk is blown off course by hurricane force winds), the teleport is randomly misdirected elsewhere on the prime plane, AND any attempt to cross a planer boundary to the approximate location of the orb results in a random teleport. This extends to all members of a multi person teleport. This condition remains for 1 year after the subject mentally gives up on ever obtaining the orb(s).
This suggests some options for the low level party start.
5)
i) they are hired to retrieve the orbs, and things go pretty well until they accidentally learn what they are, and then they have to start doing dangerous cross country travel both "there" and "back", and start racking up the random encouters on overland travel xp.. (good aligned party may then have to fight their former employer to get back the orbs they already retrieved. or the evil party figures out they have been hired by the "good guys" to get the evil orbs for them....)
ii) The PC's are about to travel using a commercial teleport service. A scholar/sage buys a ticket on the same ride. They end up in the wrong kingdom. Their "driver" leaves to consult with his guild about the problem. The scholar then burst out that :"I must be RIGHT! There IS a DRAGON ORB there!.... or at least at one of them..... ... oh, I've said to much.... ... do you really want to know... ...well now YOU can't teleport for a year either.... ...one of the possible locations is on the way to where you were going...."
iii) You don't know who you are or how you got to this table at this tavern. The letter says that your stay has been paid for for a year and you will receive instructions then. {maybe they get bored and have a few starter adventures while waiting out the year. maybe they even obtain an orb or two and sell them before getting the instructions} Then they have the "ending of Back to the Future II" cryptic letter with map delivered along with the keys to a keep and a stack of teleport scrolls. "Teleport to these locations and get what is there. Walk back to your keep as item prevents teleportation. DO NOT IDENTIFY ITEM as it causes MEMORY LOSS CURSE(or other threat).Then go to town and teleport from there to the next location. Gather all items and then go to {insert location } to have your memory restored. Signed {name of only party member wearing a name tag}"
Whether this turns out to be a "Total Recall" set up (the MOVIES not the Short story) is up to you.
Whether one of more of the PC's where shaghaiged into this (just had their memories erased) is up to you.
Whether or not some of the party were accidentally included in the memory erase spell (wait, I'm really just the stable boy?) is up to you.
6)Hell, the "Good" party could actually be the "Evil" party after a make over/polymorph/reincarnation/clone spell and will either "get their memories back and be evil again" or "fight with, be killed by, and reabsorbed into" the original body in the climax fight. The Opposing "Good" and "Evil" parties actually were not the other player groups, but actually unrelated NPCs the whole time. The Evil party got all the good orbs, but needed the evil orbs too, but couldn't get them due to "Harry Potter and the Philosopher's stone" style protection spells that meant only good guys could get them.
This way your 2 groups can THINK they are competing, but you don't have to track them crossing over each other because they are actually the same characters with different identities running 2 years apart, and the players don't know they are actually sharing characters. You can even use the same dungeon maps, just rotate them 90 degrees and swap the alignments of the guards and reskin them a little with the same stat blocks. It will be the ultimate SHOCKING BIG REVEAL.
7) After the first couple of dungeons, let the PCs choose where they are going. Some of the suggestions on the map may be wrong or out of date, or the "clue to the next orb" might suggest multiple places. Is the "Great tree" the one in the swamp, the one with the elf village, or the one with the Ewoks? But let the players decide. If they choose the wrong one..."Hey this isn't an Orb. Looks like part of a rod of some kind...." or "I thought we were looking for the Gold orb...This one is Red..." or "That isn't the Black Orb. It's black, but it is a... sphere of come sort..." or even " I didn't know there was a Crystal/Iron orb.... The Neutral dragons have orbs too???"... This gives the players more agency, and if you are doing a progressive difficulty scheme, you can insert "wrong way" side quests to provide XP and equipment if they aren't "up to level" for the next planned orb dungeon.
8) As for the nature of the Orbs...
They are a Dragon War and Dragon Genocide Prevention device. Maybe they were exchanged as "hostages" with the good Dragons holding the Evil orbs and Vice versa, or maybe neutral parties held them (psionic Crystal dragons, or Ferrous Dragons, or Yellow, Lung, or Purple dragons or whoever) or some odd combination there of. Maybe created by the gods to maintain elemental balance on the prime plan, or by a powerful wizard to stop extremely destructive Dragon wars.
i) they are indestructible, aside from one God level ritual. (they are tied to the plane and will always do their job)
ii) no Metallic or Chromatic Dragon or dragon-blooded creature of those bloodlines, nor being under the magical or psionic control of just a creature, may touch them. (got to manipulate mortals to do it) (the "dragon link" of most sorcerers are too weak to be effected.)
iii) no non native outsider may approach them. They either just can't, or are Banished if they do. Any being dominated or possessed by a non native outsider is freed upon approaching an orb. (Prevents interference with the dragons of the prime plain)(got to get a mortal to do the job)
iv) they will always find a way to be on the "surface" of the world. They can only be temporally disposed of by dropping into the ocean or a volcano. (the orbs are "available" to stop dragon wars)
V) they are protect by other wards (see above, srying and teleport restrictions) (they are very difficult for mortal or dragons manipulated by mortals/outsiders to steal)
VI) MAIN FUNCTION A: If 2 orbs are touched together, all dragons of the types keyed to the orbs with in D1000 miles feel extremely uncomfortable in each other's presence. They retreat to D1000 miles of each other and become territorial. This function is intended to break up Dragon Armies. The effect lasts for D1000 years {check separately for each dragon. ).
VII) MAIN FUNCTION B: If the last dragon of a type dies, the its orb spontaneously generates, scattered in the proper environments through out the world, 2d10 hatchling dragons of that type, at least 2 of which will be complementary genders. (prevents dragons form even attempting to organize genicides on other types, as they know that it just won't work. )
9) maybe Tiamat needs the Good orbs to destroy the world by canceling the apposing elements.... and needs the evil orbs to repopulate the land with evil dragons afterwards.
10) if the Crystal, Ferrous, Lung, or Purple dragons also have orbs, these were created later and are not nearly as powerful.
Anyway, these are my suggestions. Mix and match as desired.
