Haha! That's amusing... I just had nearly an IDENTICAL situation pop up a little while ago, but I asked on IRC, not here.
The answer is that the rules don't clearly define what to do. But here are some options:
A) Have the effects go off at the same time. This isn't exactly allowed for in the rules, but it can work. In my situation, it was a dragon readying a breath weapon, and a wizard with a disintegrate... so the wizard got toasted and the dragon turned to dust. Both happened at the same time.
B) Base it on who readied the action first. In my situation, it went like this--
Turn #1: Dragon does stuff. Wizard1 puts up wall of force, Wizard2 readies action for disintegrate. Turn #2: Dragon readies breath weapon to blast the wizards when the wall of force drops. Thus, Wizard goes first, then the dragon.
This seems to be more supported by the rules (though I can't quote said rules, since other people were giving the advice to me)... but doesn't make a whole lot of sense, in my opinion. I was ready slightly earlier, so I go first? Even if we've both been readying the move for minutes or hours on end?
C) Reroll initiative for this specific move. It's an easy way out... just shrug, roll again, and follow the results.
Hope this helps...