Seat selection has become unfortunately very complicated and in some cases also quite expensive. It also varies from airline to airline, so you need to consider this as well.
Because you need to book two different itineraries, you have to make two separate bookings. You can ask the airline to link the two bookings, but this may not be possible and it's also unlikely to help much.
If you want to reserve your seats up front, you can do this right at booking or (in most cases) anytime after booking online at the airline's website. Keep in mind that
- Some airlines only allow you to do this if you book the ticket directly from the airline (which is in general a good idea anyway)
- Some low-price tickets (Basic Economy, Economy Light) do NOT allow reserving a seat
- Even on a regular economy ticket, you may have to pay a substantial fee. For example Swiss charges a whopping 85$ (one way) to reserve a plain vanilla economy seat for something like ZRH->HKG.
If you and your boyfriend sit side by side during the seat reservation, it's typically not difficult to get something together.
The other option is chance it and wait until check-in. At check-in, you can typically select a seat for free, and if you both check in at the same time, you may be able to find something together. The trick here is to check in as early as possible. Most airlines officially open check in 23 or 24 hours before departure, but in many cases it actually opens 25 or 26 hours, so trying early is really useful. I did this last week and had no problem to get my son, my wife and myself together, since we were the first ones checking in.
Again, you'd have to check in at the same time, ideally sitting side by side.
Other good tactics are to choose for example an aisle and a window seat in an empty 3 seat row. If you are lucky you end up with an empty seat between you, and if not, you can simply trade. Everyone wants to get out of a middle seat!