I think you're guess is as good as anyone's. I doubt United understands how that works as well. I've spoken to 2 reps on the phone and both gave a difference answer. After splitting the res, 1st rep said that only I would potentially be on the upgrade list (UA Gold), which is logical. If I declined to split the res then we're declining to be upgraded.
2nd rep said that if I declined to split the res, hence we're were both on the record locator, we'd both be picked up on the upgrade list.
Trip report:
Outbound: Upgraded to First. But that was before separate the record locators. First flt on Fri to DCA. Don't know the usual loads, but seemed somewhat full.
Inbound: No upgrade. Separate record locators. Full and oversold flight in F and Y.