pvp-like battles tend to be won on initiative and huge single turn damage, don't be too worried that an advanced MU is going to be throwing bless and aid.
The issue arrises that the concentration system was ad hoc to nerf MUs so they aren't OP, but allowing only 1 hobbles their usefulness.
Let me worry about balance, I am and I have removed things that grossly benefit MUs already like shield and other rediculous spells that would basically make a mage a front line blockade.
So listen to your DM, she knows what's best for you. Multiple concentration is good. Other games allow many buffs, this game will be no different.
