Let me start by saying there are two means of obtaining these "unite" trophies. First and foremost, you earn credit for both characters on the team whenever you beat the game. This can save you a ton of time, however it's a bit trickier to use 2 characters you haven't done than it is to use 1 character you need and 1 character you're good at defeating the final boss with. This entirely depends on your preferences, however it only truly matters for the last boss fight as you can complete all of arcade mode as a different team and then lose at the end boss in order to continue as the characters you need without voiding your achievement. You are free to lose as many times as you like and the special characters don't count towards this, just the base 8.