Person A and Person B are each other's soulmates, but they're both in loving relationships with other people. Once they meet, they never want to leave each other's side, and they can feel themselves falling in love with the other person, despite still being in a relationship.

How they deal with this is up to you. Do they break up? Do they stay with their partners and ignore the feelings? Do they all start dating each other? Does someone else find their soulmate and suddenly understand what these two were going through?