Upvote:2
For starters there is no reason not to have the same flights on the return leg for both people. YOU pick your flights for each ticket and just make sure they are the same.
Seats are another story, but you can call the airline and have them tie the bookings together and request seats together. If the airline charges for advanced seating, then pay the money to be sure. Then monitor the bookings as it gets closer to flight time to make sure nothing changes.
If the airline does not offer advance seat assignments, then show up early for your flight, like three hours ahead, so you have a better chance of getting seats together. If all else fails, you can ask your row mates if they would swap seats so you can sit with your girlfriend/boyfriend/parent/whatever