If they're still doing the whole "two protagonists exist at the same time" thing for RF6, they should just keep them as adventuring bros/ farming rivals. None of that unresolved childhood friend romantic tension shit Subaru/Kaguya and Aden/Sonja had.

This way, they'd just have excellent platonic chemistry as friends that hound each other's asses. And if you do decide to take their relationship in a romantic direction, you can just easily pull the "my true love was my best friend all along" card