maybe resolving a more longstanding emotional subplot? like a really significant positive shift in any relationship is the trigger for potentially doing the friendship is magic thing
Oh, and the XP boost is available once per session per character (only one short rest tho)
OH! Maybe one of your inspiration slots is for your aspects and your other is more like 'friendship is magic' friendship gems and awarded by other players when you do stuff that embodies the Virtue their characters care about ala Epyllion?
They both spend the same, or maybe the long rest requires that everyone spend Friendship Inspiration so you can only do the big magic thing when you're all charged up with teamwork feels
I think I'd allow either in the middle of combat, too, because I love it when people shout at each other and cry and fall in love during swordfights