I need to fly SFO-BCN-SFO, and would prefer to make the transatlantic trip on United.

Ideally I would book something like SFO-FRA-BCN-FRA-SFO, with the SFO-FRA and FRA-SFO parts on UA metal, and the FRA-BCN-FRA on LH or JK or something. I can buy such a ticket from united.com. Unfortunately I'm subject to the fly america act for this ticket, and none of the *A flights into BCN appear to be available as UA code shares.

Thus, I'm considering booking SFO-FRA-SFO on United and a much cheaper, FRA-SFO-FRA trip on some other *A carrier (possibly even through united.com), and just finding some other way to pay for the FRA-SFO-FRA part, assuming it's cheap enough.

My concern is what happens in the case of a misconnect? If I've bought two separate PNRs, even from united.com, and a delay in one causes me to miss a flight in the other, will United take care of me, or will I be in trouble?

Alternatively, is it possible to book one PNR in which the segments are clearly priced separately, so that different parts of the trip can easily be reimbursed from different sources?

My fallback plan is to take DL, which does have flights into BCN. I'd do it if those flights were non-stop, but as it works out I'd have to have a stopover anyway. The United routing is much more convenient for me.