I don't know about coke but I frequently take ritalin and suboxone together and it's a nice combo, doesn't get me high but helps me function without resorting to illicit substances.
That's just my experience, I do know as a general rule you're not supposed to mix uppers and downers but I think if your using minimal amounts of each drug you'll be fine.... still best to consult a professional. I'm prescribed uppers and downers and I know a lot of other people who are as well, so I don't think it's that bad as long as you don't go crazy with them.