It depends on when you mix them. Personally I never took MDMA Exactly while still high on coke. But from what I observe from people it reduces the effects of MDMA because as someone already said Cocaine blocks the SERT. Cocaine from what I've researched increases both dopamine and serotonin levels at a 3:2 ratio so it's not as much of a 'dopamine drug' as many people assume.
But, what I've experimented is doing a fat line of good coke after some pretty good MDMA pills kicked in good. It removes the 'smacked up' feeling some people experienc from MDMA making you more speedy, plus the euphoria, empathy etc are still there if not enhanced, combined with the 'king of the world feeling' cocaine usually brings (talking a lot etc etc.) I basically got the best of both highs combined. (Of course, this may be different depending on the individual and the way these drugs work on them).
I also tried taking some good MDMA pills after coming down from about 4 lines of not so bad cocaine. The comedown was intense from the cocaine and that's the reason I dropped a pill that time. The pill still worked as usual and completely removed the cocaine comedown(though happiness was a bit less most probably due the comedown of the coke still running in the background).
Of course, Ideally you never mix the two drugs and you enjoy both of them seperately and responsibly. I avoid this combo nowadays. Especially if alcohol is involved too since another drug (Cocaethylene) gets created when alcohol is mixed with cocaine and is more damaging and cardiotoxic than all 3 drugs most probably.