I don't think that this combo is doomed from the start, but like others I have some definite reservations about the appropriateness of this pairing...
Now this is not because I scorn DXM -
au contraire. Indeed, I am one of the few incorrigible freaks who reveres that thanatotic sacrament of the nightside abyss...

...and not because I have bad taste in psychoactives; Dexter & I simply vibe together on a level which apparently eludes many - (I suspect primarily because A)its energy is shadowy and deathly, a distillation of the blackest magick; B)it is finicky and inconsistent, requiring a multitude of subtle factors and variables to align in order to fully manifest in its glory; C) the Hydrobromide confers the best results).
That being said, I engage Dexter infrequently - rarely more than once a year - because I perceive it as a morally ambiguous daemon that proffers profound healing with one hand, and corrosive delusion with the other.
If you are interested in DXM, I suggest pursuing it on its own (with cannabis). While one can learn much from a single introductory experience, it is ultimately insufficient to grasp the deeper implications of this molecule.
It is always a good rule-of-thumb to be intimately familiar with any substance that you wish to combine with another.
Intuitively I suspect that DXM & DMT are more likely to clash than synergize. They are, after all, almost perfect opposites (tho
not necessarily complementary, like yin & yang) - DMT being purest energy of Light and Life, fitting the human mind with a planned perfection... while DXM is purest energy of Darkness and Death, Absolute Zero, often producing the uncanny sensation that it was never intended for human trespass; perhaps a misplaced artifact of alien technology, or maybe insidiously seeded here with incomprehensible intent...
You sound like you have a good head on your shoulders, with proper intent & technique, etc., and I don't want to condescend or presume. But before attempting this combo I think it would be ideal if you had a bit more experience with DXM, and also if you consulted the opinion of DMT itself... does it respond with friction or openness? At least that's what I would do... attempting to ask may be more important than receiving a clear answer. Never a good idea to just blunder in... don't know whose toes you might be stepping on...
But since you seem mostly committed to this endeavor, I truly hope it is spectacularly insightful and healing and you write up a dazzling & detailed report!