It seems to be common and accepted practice that you cannot nest tickets on the same airline.

So you cannot fly AAA-BBB on ticket 1, then a BBB-AAA-BBB round trip on ticket 2, then the return leg BBB-AAA of ticket 1. (Rather the airline insists on AAA-BBB-AAA followed by AAA-BBB-AAA.)

If it matters, AAA=EU and BBB=USA.

But are these near alternatives OK?

i) Ticket 2 on a different airline to ticket 1? If both are *A airlines? What about miles? Best to credit ticket 1 to one FFP and ticket 2 to a different FFP?

ii) Ticket 2 is BBB-CCC-BBB where CCC is same country as AAA. So not strictly nested but close.

iii) Ticket 1 is revenue (AAA-BBB-AAA) but ticket 2 is award (BBB-AAA-BBB)? Same airlines? Different (*A) airlines?