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.

Instrument Upgrade w/ Split PNR at T-24
 

Apologies if this is in the wrong place - feel free to move. We've got 3 on a PNR (1 PP, 1 PG, 1 PS). Waitlisted for instrument upgrade to Business. Currently 3J left in the cabin. We've been top 3 on advance upgrade list for months (thanks to PP status of PNR originator). Question is, if we don't receive upgrade by T24, splitting into 3 PNRs during check-in is the best course of action? Correct me if I'm wrong, but if we split PNR before T-24, we'll revert to our respective status, but if split at T-24 during check-in, we'll all retain our top place (unless higher status comes along) on both advance and gate upgrade list (i.e, PP for the purposes of the list)? We're all in Business for return, so I'm not worried about lost privileges for the next flight.

While the system should work like you describe, I have recently experienced a conflicting data point.

My experience is it will not split at check-in automatically. If one person clears after check-in, it will automatically split the reservation and everyone will be reverted back to their own status which could move them down the list.

If the companions have no status, it retains the top status of the PNR before split.

Not great to hear! So in essence, if I split the PNR at check-in, there is a chance the 3 pax will revert to their respective status versus all retaining PP (for the purposes of the upgrade list)?

Yes. And you don't get a choice. If you check-in with an instrument upgrade requested, it will automatically assume you want to split if a single person can clear. If you don't check-in however it's an all or nothing upgrade for everyone on the PNR.

I feel like I've read on here that if you opt against a T-24 split, your PNR won't be added/considered for the gate upgrade list. Or that might just be for CPUs, not instruments. Any truth to that?

That's just for CPUs.

And my last question (I promise). At T-24, instruments could still be auto-processed until we get to the gate list (T-4?). So not splitting at T-24 could also result in the PNR getting skipped over (versus an agent manually processing)? More or less a crapshoot, but appreciate the insight.

Correct. The gate list is only used for upgrades at the gate whenever the flight goes to gate control (no earlier than T-1hr in most cases).

You don't get a choice. With instrument applied, UA assumes you want the upgrade.

So, safe to say PNR will auto-split at T-24 (being that we're still waitlisted for instrument at that point). Fingers crossed each PNR will retain highest status (PP) on the list? Premier agents on the phone can't even guarantee that's the case...

I could use some advice on whether to split or not and what the benefits and complications could be. Me (1K) and 2 family member companions (no status) traveling TATL in mid-Feb 2023 on R/T award (XN) on UA metal with just about all fare buckets showing 9. If I understand correctly at this point we are not eligible for CPU due to more than one companion. I have Chase Explorer card. Should I split one out to provide chance of CPU and if so what happens to the other traveler? Can I upgrade that traveler with $, miles, or +P?

Read a bit about this but haven't done it before and would rather not make a mistake.

TATL is not eligible for CPU.

You can of course purchase an upgrade for the three of you, if available. If you only seek to purchase an upgrade for the non-status pax, the PNR will need to be spilt (you can't have different fare classes on the same PNR). If you waitlist on an instrument (Miles or PP), but split PNR before T-24, your non-status pax will very likely fall to bottom of the upgrade waitlist.