I'd say it could increase the risk of Serotonin Syndrome. But who knows what the odds of that are. Definetely if you try it, start with a small dose of DXM and work your way up slowly over several trips. I would personally start with 100mg and increase my dose by 50% or less every week or so.
But then again, I also wouldn't try out the combo out of fear of a negative physical reaction. So yeah.