Well, I've heard many tales of people mixing the two and as far as I can tell, the only time you should is when you're in a safe environment (read: no stairs, or anything that could be dangerous ever) and want to get in a state where you are completely and utterly fucked up.
Also, in my case, the only person I buy weed from, I haven't talked to in a long time (like, almost a year). I haven't been stoned since before last newyears.
I may get into a little DXM sometime though... But ultimately, I'll be drinking rather than illegal drugs for now.