Multiclass caster concentration check modifier

[Randall Jhen]

So, a sorcerer 5 has a Concentration check of +5 plus Cha mod. A wizard 5 has a Concentration check of +5 plus Int mod.

What is a sorc5/wiz5's Concentration check? Is it a flat +10 plus highest of Int or Cha? Or is it +5 plus Cha for sorcerer spells and +5 plus Int for wizard spells?

I'm pretty sure it's the latter, since the caster levels don't stack.

[Morgen]

It'd be split between his wizard side and his sorcerer side. So Cha+5 or Int+5.

Combat Casting would work for both though.