PremEx, what was strange about this was that I was unable to use the multicity option on ual.com to ticket me the right way. Either I would get messages that the flights were not available, or I would get to the first stage of booking and then would get an error on the second phase. I tried to get web support to handle it but they were unable to do so. Expedia worked for some reason. And Mileage Plus upgraded me (using miles) without messing up the itinerary so I think I am looking good.

So for people looking to split the same flight number into multiple segments, I have to suggest that if one site doesn't work, another might.