I love doing this. But I never using just cannibis in the hookah. I usually will put a little (very little) amount of shisha at the bottom of the bowl. Then add a layer of cannibis. Then finish it off with another layer of shisha. Make sure you don't pack the bowl tight at all. At most, your bowl should be 3/4 full, but I usually make mine 1/2 full. Seriously, don't pack the bowl full or you won't get a good smoke going. Use maybe a gram or less of cannibis in with the shisha.

My favorite blend so far has been, blueberry shisha on the bottom, followed by some cannibis, then some pomegranate shisha on top.

Good luck and let us know how it goes!