I've always assumed it would break things to do it - but it's just an assumption. I wish someone would try out multiple concentrations in games for a while and see what happens. Not me though - I don't have time for that
I am literally wrapping up DM'ing my campaign of HoTDQ and RoT with an optional concentration rule that I came up with from day one. The party is all level 17 (I had several additional side adventures to get them to level 17) and literally yesterday are midway through the fight inside Tiamat's temple in the Well of Dragons. Party makeup is as follows: Dragonborn Vengeance Paladin, Gnome Illusionist Wizard, Human Hunter Ranger, Dwarf Grave Domain Cleric, Githzerai Open Fist Monk, Half Elf Lore Bard, Wood Elf Moon Druid.
Here is the mechanic I used for concentration:
You start the game being able to concentrate on only 1 spell per the usual rules. Once you have access to 3rd level spells you can start concentrating on more than one spell. The limit on spell level for the second spell you can concentrate on is 1/3 the highest level spell you can cast (rounded down). So once you get access to 3rd level spells you can now concentrate on any spell from level 1 - 3 and you can concentrate on a second spell as long as it is no higher than 1/3 the highest you can cast, in this case level 1. Once you have access to 6th level spells you can concentrate on one spell up to level 6 and you could concentrate on a second spell as long as it was 1/3 the level of the highest you can cast, in this case up to a level 2 spell. The effect is cumulative. So once you have access to 9th level spells you could theoretically concentrate on 3 spells. 1 spell from level 1-9, 1 spell from level 1-3 (1/3 of 9 = 3), and 1 spell of level 1 (1/3 of 3). If an effect forces the player to make a concentration check to keep a spell I had them roll for each spell they are concentrating on.
My reasoning behind this was that the higher level you get the "easier" or more "routine" lower level spells become to you as you have become so experienced at casting and concentrating on them. This goes for the NPC's as well. All my NPC casters used this mechanic as well against the party. I have run this adventure through 17 levels (obviously this optional rule did not kick in for the PC's until level 5 for some party members) and had no balance issues with it whatsoever. I have been playing/DM'ing for 30+ years across all editions of D&D (and Pathfinder) and feel that I have a good grasp on mechanics and optimization. This was even a spell caster heavy party (all but the monk) and I really felt like it added to the game play because the players would choose some still useful lower level spells to memorize/know that would never see the light of day if you could only concentrate on 1 spell at at time. The way I look at it is if I have a level 17 Wizard in my campaign and they could concentrate on a spell like Gate or Prismatic Wall or Shapechange, is them casting and concentrating on Expeditious Retreat really going to break the game? If it did then I would say I did something wrong as a DM and it had likely nothing to do with concentration. Yes I know you could also have Haste or Fly up as well, but again, AT THAT LEVEL, is either of those really game breaking either? So yes it is multiple concentration but with level limits built in under a fairly easy to remember mechanic: 1/3 of highest spell level you can cast (rounded down) for an additional concentration.
Let me know what you think about my house rule mechanic. We are about to start up another campaign and the player who will be DM'ing that has decided he liked the house rule enough that he is going to adopt it into his campaign as well (Yay! for my Hexblade Shadow Sorcerer).
[TABLE="width: 580"]
[TR]
[TD]Highest Spell Level can cast[/TD]
[TD]Highest level of first concentration spell[/TD]
[TD]Highest level of second concentration spell[/TD]
[TD]Highest level of third concentration spell[/TD]
[/TR]
[TR]
[TD]1[/TD]
[TD]1[/TD]
[TD]-[/TD]
[TD]-[/TD]
[/TR]
[TR]
[TD]2[/TD]
[TD]2[/TD]
[TD]-[/TD]
[TD]-[/TD]
[/TR]
[TR]
[TD]3[/TD]
[TD]3[/TD]
[TD]1[/TD]
[TD]-[/TD]
[/TR]
[TR]
[TD]4[/TD]
[TD]4[/TD]
[TD]1[/TD]
[TD]-[/TD]
[/TR]
[TR]
[TD]5[/TD]
[TD]5[/TD]
[TD]1[/TD]
[TD]-[/TD]
[/TR]
[TR]
[TD]6[/TD]
[TD]6[/TD]
[TD]2[/TD]
[TD]-[/TD]
[/TR]
[TR]
[TD]7[/TD]
[TD]7[/TD]
[TD]2[/TD]
[TD]-[/TD]
[/TR]
[TR]
[TD]8[/TD]
[TD]8[/TD]
[TD]2[/TD]
[TD]-[/TD]
[/TR]
[TR]
[TD]9[/TD]
[TD]9[/TD]
[TD]3[/TD]
[TD]1[/TD]
[/TR]
[/TABLE]
Worr