Interestingly, I asked a phone agent today, and she said the opposite--i.e., that UA will NOT honor the free SDC after the split. Sounds like it's going to be a gamble.
I split a PNR at check-in (which is admittedly different in a few subtle ways) and had no trouble getting both PNRs subsequently SDCed. The agent had to process it as two separate SDCs (and managed to drop all my flown segments in the process so I had to get them manually credited..), but was able to do it all on one call to the 1K desk.
The agent can certainly tell that you all used to be on one PNR; perhaps a HUCA situation for an advance split?