be aware that theoretical seating will make getting seats together across separate bookings more difficult during online checkin

3 days before the flight ( 2 before checkin opens) you will see a very different seat map as seats will get blocked and only limited seats will be made available. when you check in people on one booking the seats beside them will be blocked until more people check in and those seats are needed. seat blocking does not only happen for gold!! the higher the status level of the checked in passenger the longer the seats will remain blocked

so if you chose not to purchase seats then you may be best to check in the 2nd party in seats near the other booking, but not download and print the boarding pass, then keep checking and see if "better" seats open up