Like Thou I've mixed 2C-E + DXM. I took a regular dose of 2C-E (10mg or 15mg, I can't remember) and a low dose of DXM (100mg) and they both potentiated each other very much - I actually felt significant effects from the DXM which surprised me as I was told I wouldn't feel such a low dose, though it was still weak. The 2C-E felt more like 15-17mg and provided a very strong and fun trip, though I drifted off and lost track of my thoughts a lot more than I do on 2C-E alone.
I enjoyed the combination very much, and it's worth noting that unlike Thou, I dosed both chemicals at the same time, so maybe that was more appropriate timing.
Bear in mind I was also smoking a lot of weed that trip.
My advice here will be the same advice I give for pretty much any combo (aside from obviously unsafe ones that shouldn't be undertaken) - be cautious, start very low with the dosage since they'll likely potentiate each other, and of course, don't add any other substances into the mix.
I'd suggest something like 150mg DXM + 20mg 2C-C just as a starter to see how well you react to this combination, then you can raise your doses next time if all goes well
