Hopefully you can get a better (correct) answer from the pros in here. However, my experience with a split PNR was recent. I believe after the split your wife keeps your status on the waitlist, at least that’s what happened to us. Probably because we applied the MUA before the split?
Initially 3 people on one PNR. Applied a MUA for all 3 to our inbound flight. Later we split the PNR so my wife and I could upgrade on the outbound flight (not enough miles for 3, $ neither ). On the inbound flight my daughter remained by herself on the split PNR (she has no status) but she was actually ahead of us on the wait list, but all 3 of us were together. I’m Gold and wife is Silver.