A truly interesting combiantion!
I tried that combination to great effect recently... the first time i tried mixing a tryptamine psychedelic with DXM. I took ~525 mg DXM approximately 2.5 hours after ingesting approx 350 Morning Glory seeds with several pills of meclzine to cancel the nausea (why people don't use this compound more regularly as an anti-emetic always escapes me). Once the DXM took effect, it took over the character of the trip completely, but with the bright complexity of morning glories in effect, which, among other things, added color to the DXM visuals, made them much better defined,crystal clear, in fact, behind closed eyes, which for me is rare on a dose of DXM lower than 650+ mg. it was also very disorienting, and the morning glories seemed to augment the post-experience amnesia caused by DXM, leaving me with he impression of intense mental activity that I could only recall a few episodes from, as with many of my DXM experiences. It also had a slightly darker quality than my average DXM experience, as though the DXM had somehow "manipulated" the power of the morning glories in ways that caused some rather nihilistic revelations, as though I was intruding on somewhat forbidden territory. It was very interesting, despite the slightly forbidding atmosphere it created, but there was a moment of very painful nausea which subsided quickly, after one draw on some good bud. A combination that is well worth repeating, but which, as with any psychedelic, and DXM in particular, requires correct mental/psychological preparation, in order to not be taken too much by surprise. It was, in many ways, reminiscent of the combination of DXM and a high dose of good quality cannabis, but much, more complex in effects, and with none of the short-term memory deterioration ("brain fog") inherent to cannabis.
Also, no indications of serotonin syndrome noted; I believe I have experienced that before after unwisely combining 400 mg DXM with 200 mg 5-HTP and experiencing extreme muscle tension (particularly in my neck and arms) highly toxic, unnatural nausea, fluctuations in perceived body temperature (feelings of heat and cold), and for a short time, substantially scattered thoughts. Morning glories caused none of those symptoms, and was an experience that, with more adequate preparation, I intend to repeat in the future.