In my experience that is the one that would be possible, but in practical terms only if you had 2 bookings in the same name on the same flight - one may get cancelled as a presumed error. Even then I have had 2 bookings on the same flight hold up to the gate quite recently, the only potential impediment would have been that 2 passengers can't have the same passport number where there is an APIs check. Some people have more than one passport....

The other one reported here, but which I regularly fly without issue, is 2 flights on the same day on the same route, which for short domestic hops is perfectly feasible. Apparently they can get cancelled, but it's something I do fairly frequently, without issue.

The OP is doing 2 different destinations, on 2 different reservations, which I've also done and not had an issue. The only difference being that my unflown sectors would have been inflexible, and I don't believe BA's computers are clever enough to go through the fare rules and treat that differently.