The best you can do in this area is to both travel Plus in one direction, and both travel Basic in the other direction.

One big aspect is that Same Day Change is only on Plus tickets. Now if there is only one flight a day to a location, which will apply to many LGW destinations for example, then that's not much of an advantage. But otherwise it is worth paying the extra £10 or so for both baggage and SDC potential. Also AUPs (but not POUGs) are sometimes cheaper for Plus rather than Basic.

The usual phrase used for an Emerald to get someone manually seated in separate bookings is TCP.