I've had a number of experiences of this over the past year and every time it is an issue.

First off, splitting the record messes up a bunch of things. Like I got forced overbooked on a new routing when my W to E coast flights got delayed. My young son's did not as he was on a different locator and had no status to get such nice treatment - I was low on 500s so I had put him on a regional upgrade. In the end the gate agent did something she should not have done for sure as that alternative flight was way way overbooked. (It did illustrate the advantage of being a 1K and being nice to gate agents when everyone else is shouting at them). Then there are other things like the other person has no status so with any upgrades you get upgraded and they have no hope. And if you are both standing by for upgrades on a subsequent flight then you stand a good chance and they are hosed. It kind of goes on and on - and the worst thing is not failure to get first - it is getting stuck somewhere with no status if flights go haywire. I cannot recall the number of times when I have got on the last flight out of somewhere in bad weather leaving tons of people at the gate - my travel partner would be stuck if we were on different locators - and that does not work with a kid.

You can use any combination you like at the gate or at check-in of course - provided the agent knows what they are doing. But that assumes that first is pretty much wide open still.

They won't fix this for sure - it is too esoteric.