I think, if you wanted to add a dissociative with a psychedelic, you should prefer mxe or k.
The first is combining well with psychedelics, accentuating the "essence of the drug". K is... fastest, sharp and clear.
The dxm would be too handicapping for me, and if the trip turns weird and ugly, you should be prepare for a rather long trip. But I don't know your experiences with this drugs.
Watch out for the serotoninergic syndrome too ! and sorry for my bad english..
edit : one monther and half later... sorry.