Thanks for the reply. Can you explain? Are you saying United won't let me book overlapping reservations that are impossible to complete?

Makes some sense. Although if my flight home is on a Saturday, and my outbound work trip flight is on Thursday, it's technically possible for me to be back to make that flight on Saturday. Not likely, and of course really unlikely if that work trip itinerary has another leg after that Saturday, say the next Tuesday. Haven't booked the work trip yet, but good idea to use a different airline and avoid the chance it gets flagged. I'll miss the mileage, though.

​​​​​​I wonder if what manner they enforce this. I guess they could cancel my ticket home if they believe I'm not even in that state. That's what I would want to avoid. If they enforce it by denying the purchase of the work trip, that's not such a big deal.