I've heard about this in another topics.
As far as I know, when you have more than 1 critical strike, they work separatedly, means that every hit checks the % of both independently, but if both critical strikes happens at the same time, only the lowest one works, even when both damage amounts are displayed as criticals.
PlayDota community has some information about Wc3 mechanics.
http://www.playdota.com/mechanics