I always thought of fencamfamine as a dopamine reuptake inhibitor--like methylphenidate, prolintane or ethylcathinone, it is a bulky N-substituted amphetamine, right?
Anyway, to the original topic, combining dopamine reuptake inhibitors will just get you more uptake inhibition, with the added cardiac strain and arrhythmic potential of cocaine. Doesn't seem worth it and sounds possibly dangerous.
It's a damn shame they discontinued Reactivan and other fencamfamine products. While I've never tried said compound, it seems like a wonderful mild-ish, smooth-feeling general purpose psychostimulant--like a side effect free methylphenidate or a faster-acting, moire euphoric (and unfortunately, I suppose, reinforcing) modafinil. And with Reactivan, apparently they threw in some free B and C vitamins as well (which can't be a bad idea for stimulant users).