I just have to point out that this reasoning doesn't quite gel to me, as this sounds like a case of the people at the party mixing weed and crack, not a drug dealer mixing the two.
All the legalization in the world is not going to stop the consumer from mixing any drugs that the wish to mix.
I agree but I'm talking about the sort of regulation they have in Amsterdam where you can only purchase directly from licensed cafe's and the stuff is not allowed to be freely distributed at parties by random strangers. Obviously people will slip through the cracks as with any law but it is going to make it far harder.