On top of it, oGCD abilies do not trigger the GCD, meaning you *can* cast them in tandem. Unless your reaction time is so bad you can't release one key a fraction of a second before the other. In that case I can't help you.

Then again, I also don't macro them and have the oGCD queued so it goes off the instant the cast finishes.

My point being, you don't need a cast time shorter than GCD to weave.