I read a study recently that was published on PubMed about MDMA having MAOI activity in mice (
https://www.ncbi.nlm.nih.gov/m/pubmed/7945733/ ) which means it's highly probable that it has the same effect on us. Some anecdotal evidence to back up the idea is the experience of smoking DMT while rolling. Rolling will perhaps make the DMT experience more powerful, but rolling alone shouldn't make the DMT stick around in your system for longer and continue having effects past its normal 5-15 minutes- MAO inhibition would have to be involved for the DMT to actually last longer- and people who have used DMT while rolling say exactly that, that the DMT lasts longer. Much longer. They've often reported it lasting for hours sometimes- between an hour and I've heard reports of it lasting several hours in some cases.
Mixing DXM with anything that inhibits monoamine oxidase (which MDMA DOES DO to some extent,) is a very bad idea that can lead to very serious complications including death. This is not the game you wanna play!
I'm not sure whether the inhibition of MAO-A or MAO-B is the more important one to watch out for in the case of DXM use, but while MDMA does inhibit some of the MAO-B in your system, it has a selective potency for inhibition of MAO-A and it's much better at inhibiting the A-variant than the B-variant (for comparison, the harmine in Ayahuasca, for example, selectively inhibits MAO-A but not MAO-B, [harmine is actually a RIMA, to get technical, which is a 'reversible inhibitor of MAO-A',) which is why there's a suggested Ayahuasca diet but it's not as 'life-or-death' mandatory as the MAOI diet for pharmaceutical MAOI's.
In short, MDMA has MAO-inhibiting properties and should 'never' be mixed with DXM.