No
no, it only works on a per caster basis.
Meaning only one entity can fire the proc, and cannot fire it again
However, if it's in say, multiple Single-Target attacks, you could keep proccing on multiple targets as long as you switch targets.
Or, if you have a pet, it can proc it along side you, so you can have 2(or more..) stack
Does anybody know if you have achilles heel slotted in multiple powers does the proc stack?