How do you bill an Auto Insurance and Medicare for the same Patient for the same Episode?

When I asked about billing Medicare and an Auto Insurance for the same patient for the same episode, I meant I needed to know how to bill them at the same time after discharge as "Primary" because we do not know if the Auto Ins which is billed per visit has been exhausted  by the time our claim gets to them and will deny.  Medicare has to be billed timely (Rap & Final) and that would be Primary if the Auto Ins denies. Please advise. Janet