I did some tests last night with Twister Proc +125% and Meteor Proc +135% with Prismatic Talent (so that I could see if there were more than one Proc at a time).
when using Comet with Twister Proc and Pierce with Meteor Proc, it looked like I got a Proc every time the Cooldown finished and a chance for a second Proc before the Cooldown was done with the 100% Proc. it was really nice!
when Casting Twister with Twister Proc and casting Meteor with Meteor Proc, since Procs have the same Cooldown as the original Skill, the Proc Procs at the same time as the Skill is cast. the problem is that I didn’t see an extra Proc for the over +100%. because of this, I don’t know if I was getting the extra Proc but the animation doesn’t show it or it wasn’t working.
my thoughts on this is that it looks like you get the extra Proc when using MH Primary Skills (I didn’t test with OH Primary Skills) but because in Patch 3.0 the % chance for a Proc every second was changed to % chance when Cooldown is finished, when using Special Skills to cast Procs, you don’t get the extra Proc. also, since Procs are affected by Cooldown and Skill effects, some Procs can have a Cooldown less than a second.
some Skills wouldn’t be able to be double cast anyways, like Storm, which can only have 1 at a time. if more than one is cast, the first is overwritten by the second.
I’m not sure how accurate my tests or thinking is on this subject, but until a Dev says something, 100% or less for any Proc to be on the safe side for not wasting space on our Equipment.