It is what Sean said.
I have both Medicare, A&B, and Tricare for life.
Any medical bills are first paid by Medicare, then sent to Tricare which pays the balance. You will be out of pocket, nothing.
There was a tripping point that caught me. The doctor's office needs to know to send the request for payment first to Medicare, and then to Tricare. If they do it the other way around, you will get the debt collectors calling. I would have thought the billing department would have known that, but as with all things these days, intelligence comes at a premium.