Can't remember the name for the life of me, but there was an isekai where 2 rival gamers ended up in the same game and not only stayed monogamous but jumped to an alt universe to help their alt selves get together. The entire story was basically just them flirting with each other the entire time as they saved the world. Great story.
Edit: It was Ending Maker. The Regressor and the Blind Saint is another.