Yeah it should be fine but again like the above user said don't take it with ssri's and anything else that messes with your serotonin receptors.... Do research on erowid whenever you are unsure of what drugs not to take with one another or even here on bluelight (talking about the reactions one may have mixing drugs and yes I understand xanax w/dxm hasn't been talked about, just giving this advice for any other drugs you're unaware of). In the end I would of just taken the dxm by itself because there's no need for mixing the two... I mean dxm should have you feeling good without the benzos and sometimes taking benzos while tripping kind of kills the high of the other drug (making you drowsy ect... Not as aware of the other drug taking affect). You should be good though combining the two... Just do not mix anymore drugs/medications with it and especially do not mix alcohol into the equation. Good luck even though you probably did this already by now.
You could also dose the dxm first and then decide if you still want to add the xanax into the equation but don't pop 3mgs right away. Break up the dosage and redose if needed.
dxm, benzos, and weed take my advice...