Two criticals can not trigger at once, their chances however can roll both at the same time. Basically if one chance to crit fails, the next one will roll and so one. Many people fail to realise this in maps as it can be majorly abused to give 75% crit chances.
Basically having 6 items with 15% crit each and a 25% crit from a main ability gives you a 72% chance to roll a critical strike. The multipliers do not stack in any way and so the multiplyer used is whichever roll procs.
While I am at it, evasion does not stack in any way and always the largest evasion ability will take the rolls for an evasion (as if it was the only one).
As for your second question, add the ability to the unit and then detect when it attacks the unit. As soon as it attacks a different unit (starts to attack), you remove the ability.