I've done this mix before, not sure the dosages. The yohimbe was from a male enhancement pill and i only took half, i don't think it listed the dosages. I know at the time i was snorting small lines of crystal every couple hours, and after taking the yohimbe.
All it really did in terms of negative effects was increase heart rate and made me sweat more than usual, which lasted mabye 30-40 mins. I only noticed the negative effects about shortly after a 2-3 hour sex session. I also couldn't cum at all, which made her a little frustrated, lol.
I would be careful though, as its probably not a good idea to mix two drugs that increase heart rate like that. If your using it for sexy times, the risks arnt worth the rewards IMO.