Can you mix Advil and xanax?

YES mixing them will definitely be more dangerous than taking either separately. The LD50 (fatal dose) of either is decreased when taken together.

the most important thing is your own tolerance for both of them. make sure each is safe for you separately , and then take the minimal dose of each together your first time.

DO NOT take more because you think the effects are not strong enough. that IS dangerous. take it once and ride it out, and then take a little more the next time. otherwise you are being stupid and risky with your life.