Stacking Percentages?

I know this has probably been asked before, but since search is disabled I'm outta luck

If I have a 10/10 wand and a 20/20 off hand, are the percentages cumulative for a combined 30/30 on a single roll or is it sequentially rolled, first the 10 if that fails, then the 20?

It's sequential, but whether or not the 1st fails, the second is also rolled, giving you for possibilities.

no effect
effect of wand
effect of offhand
effect of wand and offhand (not stacked, so 20/20 + 10/10 would be 72% of normal speed rather than 70%, 20% + 10% of remaining 80%, or 20%+8%)

It is sequential; it "rolls" each chance separately, so the percentages are multiplied, not added. So in your case:

No effect: 72%(both chances fail)
Half Cast or Recharge: 26%(one chance succeeds)
One Quarter Cast or Recharge: 2%(both effects trigger)

http://www.guildwarsguru.com/forum/s...d.php?t=119826

Is your new best friend. I actually posted in there about what happens to a 10% + 20% bonus

Perfect, thanks.