There is no major reason why combining Xanax and pills should be dangerous, though taken to excess they can mask the effects of each other, making you believe you are more sober than you actually are. I.e don't take 4 pills and heaps of Xanax and think you can drive OK because they "cancel each other out" (they don't, you might just feel like they do). Combining drugs is much more risky than doing them by themselves, so always make sure you let a friend or two know what you've taken so that they can call for help should the situation arise.