The most obvious example here is Kingofdapirates' "Maggie/Amber" series. It's a fantastic series but them becoming lovers is something that happens over several stories, not one.

My "Inge/Conchita" stories are kind of like this, though the women never stop being rivals. I am currently working on a story that is kind of like this, but it is far from done. My story "New Neighbors II" touches on this.

I am sure there are a lot more, maybe even some I wrote, but nothing else leaps to mind right away.

JB