It depends on your tolerance, but with information provided I'd say don't bother. That's a fair old dose of codeine, you wouldn't gain much adding xanax to it. And worst case it could affect your breathing.
If the codeine alone isn't enough for you then you need evaluate where you go from here. Adding benzos isn't ideal, lots of nasty side effects. You could try a sedating antihistamine like promethazine. Ultimately your gonna have to move to stronger opiates or cut back, you won't get much more mileage out of codeine.
Good luck, hope your ok