Can you do this?
After reading the PRD on readied actions I want to say no, but I'm not sure.
Just so were clear, Scenario:
mage A: readies a damaging spell (say fireball)to be cast when mage b casts a spell.
mage b: starts to cast spell
mage A: readied spell goes off
mage b: takes xd6 damage and has to make a concentration check to finish the spell
So any thoughts?
Can you do this?
Yep.
Ready - Distracting Spellcasters: "You can ready an attack against a spellcaster with the trigger “if she starts casting a spell.” If you damage the spellcaster, she may lose the spell she was trying to cast (as determined by her Spellcraft check result)."
Though they probably mean "concentration check" not "Spellcraft check."
Injury: If you take damage while trying to cast a spell, you must make a concentration check with a DC equal to 10 + the damage taken + the level of the spell you're casting. If you fail the check, you lose the spell without effect. The interrupting event strikes during spellcasting if it comes between the time you started and the time you complete a spell (for a spell with a casting time of 1 full round or more) or if it comes in response to your casting the spell (such as an attack of opportunity provoked by the spell or a contingent attack, such as a readied action).
Spell: If you are affected by a spell while attempting to cast a spell of your own, you must make a concentration check or lose the spell you are casting. If the spell affecting you deals damage, the DC is 10 + the damage taken + the level of the spell you're casting.