It is true that your body can build a tolerance to narcotic medication taken over time.
I've taken oxycodone or Percocet on a pretty regular basis since a few months before my first surgery. I backed off some a month or so before my second hip replacement--only taking it once a day if absolutely necessary to get me through a particularly painful day--so that it would have some Ooooph again when I had the 2nd hip surgery. Worked fine after surgery. I was taking the higher end dose for the first couple of weeks after surgery (10 mg every 3 hours) and I did notice that it seemed like I was building a tolerance again, but it still worked on the surgical pain OK at that dose and frequency. I think my surgeon would have upped it higher or added something else if I had needed it. You absolutely need good pain meds after surgery even if that means you are taking more than you/they would like for a short period of time. You need to be comfortable and able to move.
Are you still taking narcotic meds from the first hip? If so, how often and what dose? If you aren't needing to take it regularly at this time through till when you have the next hip, I think you will be fine going into the next surgery and the med will work. You may have to up the dose a bit depending on how your body responds, but generally it should work. Did your surgeon say what he plans to do if the med doesn't work well? That's something to know going into the second surgery. If he is doing your surgeries this close together he must do the same for others and should have a plan in place.
Dorothy