This wasn't my experience when traveling earlier this year with my fiancee. We're both Silver and were boooked under the same PNR. When it came time to being upgraded, we were upgraded as we appeared on the upgrade list. I managed to get the upgrade, but hers did not process. You shouldn't have to worry about splitting anything, it will all depend on where you fall on the list, not the fact that you're on the same PNR. You're against one another, and the ticket that was purchased first in the system will be higher.