Here's my question..

I Have a return flight JFK-FRA-DME.. I wanted to chng my ticket to make a stop in FRA and purchase a separate ticket elsewhere and then continue FRA-DME a week later.

Problem is that my fare allows date change but no stopover..

The other destination I want to purchase a ticket to will also be with LH

Can I check in my bag from JFK to FRA (I have a long stop over 6 hours so I can say I'm delivering it in FRA) and then checkin for a different LH flight in fra while I already have one boarding pass continuing to DME? I do have two passports if that would make anything easier..

Would the checkin counter notice I'm checked in to DME and not allow me to check in to my new flight?

Any other issued I may encounter?

Will having my united mileage attached to my new booking raise some flags being that the same mileage # has a boarding pass already issued?

Thanks