Upgrades on a single PNR are for all on the booking, or none on the booking, and based on the status of the pax who made the booking. The highest status pax should book and request if that’s what you plan to do. Exception to this I believe is if you have a P1 on the booking and they request the upgrade.
Separate bookings are treated separately based on the status of each pax. The linking of bookings will share the boarding group of the highest status pax to the other, nothing else obvious will happen (you will appear as travelling together in the system is all)
Booking would need to be split for travel movements to be altered for one but not the other. A new PNR will be created when this happens, but anecdotally and so long as the new travel dates are still within the eligible travel period for the DSC then it should work out for both.