score:3
It really depends on the airline you fly with, as there is no one rule fits all for seat assignments.
Some airlines you can select your seat at booking. Some airlines you can log in after booking to select your seat. Some airlines you can pay to choose your seat. Some you just get what you get.
With the first three, you both simply have to do your travel arrangements at the same time, so you can see and choose two seats together. And you need to accept the fact that most seat requests are just that, a request, not a guarantee. So you have to monitor your reservation frequently to make sure no changes have occurred, as airlines do change seat assignments for various reasons.
With the "you get what you get" option, the best bet is to arrive at the airport early (like as soon as the flight opens usually 3 hours ahead) so there are the most seats open for the check in agent to choose two together.
And of course in worst case scenario, you ask your seat mates if they wouldn't mind switching so that you can sit with your girlfriend.
Upvote:0
This ought just to be a question of making sure you're both booked in the same cabin class (and, of course, on the same flight), and then checking in together when you reach the airport.
There's no rule saying that passengers can only sit next to each other if their tickets are bought together or otherwise "connected" before the flight. (Though a lot of people traveling alone would probably welcome such a rule if it would guarantee them having an unoccupied seat next to them :-) ).
Depending on the airline, there's also a good chance that you'll be able to choose your seats online before reaching the airport. In that case you can simply choose seats next to each other at that time.