I think there are multiple interactions that make combining MDMA and DXM very stupid:
- risk of serotonin syndrome
- enzymatic inhibition which further increases MDMA risks and side effects
Cocaine is a CYP2D6 inhibitor which is responsible for DXM to DXO conversion IIRC.
I think you would be risking overdosing on DXM as well as getting hypertensive crisis from cocaine (or the combination) and cocaine also has serotonergic effects (although perhaps most of that is reuptake inhibition) and that is I think also to generally be avoided with DXM.
Why risk your health if you say you aren't even such a fan of cocaine?

You mean it is an alternative option to MDMA for you? I really wouldn't.

I was about to say: why don't you just combine it with something much safer if you must, but the fact is that many other drugs fuck with DXM metabolism or make you very dizzy, etc. It does not combine well IMO, but you ought to check the William White DXM FAQ.
My advice with DXM is stick to the weed and just up the dose if you want to make it more interesting.
Or: Seriously just get some ketamine instead if you want to do combine a dissociative with uppers. Though I still wouldn't pick cocaine because the effects on your 'ego' are virtually opposed to one another.