Then gestalt would be the best bet. Though it varies for any given encounter and the party make-up, I still hold to my loose guestimation that gestalt is like a +50% power boost, so a party of 4 gestalt ~= party of 6 regulars. Again, not exact at all. For example, with less bodies, affects like mind blast can be far more devastating, though they'll have better saves to resist it, too.
Gestalt basically just magnifies the facets of 3E even more so. With two class progessions, system mastery and powerful combos can become even more prevalent, for example, leading to even larger disparity between two PCs. On the other hand, I think you'd have to purposefully try to make a bad gestalt combo, so those less experienced players will also feel useful nearly all the time, just from sheer weght of options. Some things I've learned about gestalt, and things you should consider:
Multiclassing. Two progressions means far more opportunity for level dipping and cherry picking. For my game, I limited the players to only their two gestalt classes. That should give enough versatility for a character concept. They wanted a little more, so I also allowed a single prestige class.
Existing multiclass feats / prestige classes. I banned outright any dual progression prestige class (mystic theurge, arcane trickster, etc...). They're not needed, as you can just be the two classes now. Feats are a bit trickier. I basically just put out a general "no level stacking beyond your HD" rule. So a sorcerer//monk could still take that muliclass feat and get charisma instead of wisdom to AC. But the levels would only matter for monk AC if say...the player went into a prestige class on the monk side. Then the sorcerer levels would fill the gap.
Effects of LA. In a game where every level gives more, losing a level to LA is even more damaging, IMO. That said, I don't really like buying back LA, either. So I adopted a system similar to E6's, where players w/ LA start with a lower point buy. Racial HD take up one side of the gestalt, which means they don't hurt so much (other class can largely override low HD, saves, BAB, or skill points).
How to handle level ups. Basically, there are four main things everyone gets at level up: BAB, HD, saves, and skill points. You have to be careful how you handle them. I chose to split how I handle them. For BAB and base saves, the players tally up what each side would give at each level up (easier than it sounds, it should almost never change which side is better) and take the better one. For skill points and HD, they just take whatever's better at the current level up, regardless of the prior levels. If you were to have BAB and saves also work like that, a player could use racial HD or another (prestige) class to stagger the HD/saves. Ie, a Fighter 1 / Sorcerer 19 // Wizard 20 would have full BAB.
Don't use a single powerful boss. This one surprised me at first, but yes. Any big fight should have a group (or even enemy party) of NPCs, all formiddable to the PCs. With the benefit of two classes, at least in my current campaign, nearly every single PC has one or many ways to utterly devastate a single target. Thus far, I've managed to make only one fight against a single NPC work (he had a powerful animal companion, but it got chased off early on by fear effects anyway). The fight lasted 6-8 rounds (I didn't keep count). Of that, the PCs wasted 1 and a half rounds attacking an illusion of him in the air (they were rolling poorly). After that, he stayed alive because he had super tricked out AC (legitimately) and buff spells. The PC cleric also threw up a darkness spell, which lengthed the combat. Even so, it took just three attacks to fell him. He was level 6 gestalt (same as party) and had full hp. About 70, I think.
Don't let the party rest. I'm lucky to have a group that does't like to fold it in early unless things went really badly. But if your group would find the "5 minute workday" attractive, ban the spells that allow it, like rope trick. To me, one of the major points of gestalt is the nearly unlimited resources the party has, so I like to push them on for much longer dungeons than a normal party could handle. This isn't a huge balance issue (though it's easier to abuse than a normal party just from extra spells available), but I find it one of the more fun things about gestalt, so it'd be like a missed opportunity.
Balance within any single combat. While the party can go on through many fights, in each single fight, their hp won't be much higher than a regular party. So the way to challenge them should probably be less focused on attrition, and more on making most of the combats close to the limit of what they can handle. Do note, at least IME, the extra resources means the party might end up in a very difficult fight, instead of playing more defensively, absolutely nuking the opposition with everything in their arsenal. This tends to happen when my party gets beat up really badly in the first few rounds, so I try to go for enemy attacks that do less damage, but hit more reliably (or always, like magic missile), to wear them down more slowly and keep their twitching hands off the launch buttons.
