Just save the dxm for another night because the xanax will dull your desired effects. Im aassuming youre talking about dextromethorphan polystyrex?(delsym) It will make it so you have to take double the dose to achieve desired effects, BUT you will get hit with the same dose 6 hours later, it's been proven if you space out taking dxm more dxo is produced and the high is more dissociative, if you take it all at once it's more of a hallucinogen, some food for thought.
Yes, it'll be more like you start to come down a little and before you know it youre dissociated heavily because the second dose is being released from the polystyrex