Each carrier carries their own file limit guidelines (How long you get to file)
Some are 90 days, some are 2 years, and everything in between.
When there are 2 policies, you get usually 60 or 90 days to mail the claim to the 2nd carrier with the EOB from the primary payer.
If the claim is lost in the mail, misrouted - or whatever the carrier is telling you - there is a problem.
See, the thing is - you probably signed a piece of paper at the hospital guaranteing payment to the hospital for any and all services rendered.
This puts you responsible for the bill. The hospital send the claim, but ultimately it is the patient's bill.
I see this all of the time.
Only a truly happy person can enjoy the scenery along a detour