Kris and Noelle basically grew up together as a married couple.
They knew each other's soul to a T.
Eventually, though, they looked for greener grass and separated.
As a twist of fate both Kris and Noelle see Susie as a potential new romantic partner.
Eventually they realize what each other feels for Susie and rekindle their childhood romance.
They get back together along with Susie to make the first successfull main stream throuple.