I can confirm that my initial split reservation did not negatively impact my companions' placement on the upgrade list. In fact, amusingly, the PNR with the two non-Premier members is actually ranked higher than the PNR I am on! Worth noting is that the UA agent left two companions on the original PNR but split myself and one companion onto a new PNR with a new confirmation code. My best guess is that the other PNR is ranked higher based on time of request, as the new PNR is considered to have a later request time (i.e. the time of the split).