Mandatory characters

It looks like the subroutine at CA/CB25 is called to identify the characters you can add to the party (CA/CBAF is also called on other occasions, but that seems to have other functions, as well as restoring the party to full and recovering their statuses).

So for your application, I'd try to call CA/CB25, empty the party, then add your two mandatory characters to the active party and call your party selection screen and see if it works. Pretty much just copy the code from CA/AB51 down to CA/AB67, changing the character IDs as needed.