Multiple Instances of Damage vs a Caster while casting

[RAWmonger]

Like the title says, how do multiple instances of damage while a caster is trying to cast effect his concentration check dc(s)?

For instance, if two players readied an action to attack him if he started casting, they both hit, they both did 10 damage. Is the dc of the concentration check at 30 + spell level, or does the caster make 2 separate checks at 20 + spell level (Is the damage from all sources added, or is one concentration check per instance of damage?)?

Looking for RAW hopefully on this.

[blahpers]

For reference

The concentration check occurs as a direct result of the damage being taken. Thus, just as being struck multiple times by a poisonous bite results in multiple saving throws, being injured multiple times while casting results in multiple concentration checks.