From my own experience, I wouldn't do it...they're both uppers, first of all, so it's just going to be worse for your body than either one taken separately...further, though, I get totally different highs off either one and I just can't see coke mixing well with speed. The euphoria that each drug gives you is somewhat similar, but, I don't know, the main reason, in my opinion, that you'd mix coke with something is to just get extremely alert and have all your mental energy focused in, like, 1-2 hour bursts. With speed already in your system, that extra focus and mental/physical energy that you get from taking coke would pretty much go to waste. But if you're really into that alert/energetic feeling, I guess you could possibly dabble in low doses of mixing both.
I'll admit I've never tried this combination, because I thought it'd be a waste of time/money, but I've done experimenting with both coke and speed with different drugs...the closest would be speed and caffeine (5 hour energy shots), which was an absolute waste of time. Instead of doing the work I set out to do, I was just buzzing non-stop for 5-6 hours, it was an overall negative experience, and I had a pretty bad come-down - my heart was racing like a motherfucker...
If I was going to mix them with anything it'd just be weed, but I'm into feeling trippy and alert at the same time, so that's just me. Or sometimes coke after a few beers goes well...but I can't really see any positive to mixing coke and speed.