I doubt I'd do it either. You dont find your stimulated enough from the clen? Might be time to up the doseage...
Clen and ephedrine both target beta receptors. No real point in taking them together. I'm pretty sure clen targets beta2's and ephedrine targets beta3's, but someone can correct me who knows more about it.
All in all I'd probably stick to caffiene for a pre workout kick, and save the ephedrine for your 2wk off time from the clen.