Yeah, it seems risky, mostly because MDMA is a pretty powerful vasoconstrictor and mephedrone is even more powerful. You'd be risking synergistic vasoconstricting and hyperthermic effects. Mephedrone doesn't seem different enough from MDMA to me to really warrant a combo. Who knows, maybe those who report its a good combo would have had just as good a time taking a roughly subjectively equivalent additional dose of just MDMA or mephedrone.
I certainly wouldn't recommend it, nor would I do it, but if you're so inclined it'd probably be more interesting to try modulating the high of one or the other with a selective monoaminergic drug like MDAI (for serotonin) or a selective dopamine re-uptake inhibitor, etc.. That way you could theoretically find a sweet spot where altering the dopamine/serotonin balance of either mephedrone or MDMA results in an improvement (granted it's more complicated than this, but it would nevertheless be more control than you have mixing two multiple monoamine releasers.