1)Shining and Cadence always had a very healthy and open relationship, they love each other a lot, but have always been open to share that love with other ponies, even as teens, when they confessed to each other that they had a crush on their class partners, Cadance had a crush on her fencing teammate Fleur de Lis, and Shining had a crush on his etiquette class partner, Fancy Pants. They often teased each other with little comments here and there.
After many years turns out that Fancy and Fleur get together, and they encounter Cadence and Shining at the Grand Galloping Gala and start to chit chat about their teen years, Cadence and Shining blurt out in a playful way that both of them had a little silly crush on Fleur and Fancy and they are like: wAIT YOU TOO?! To their surprise, turns out that they also had a crush on them, and Cadence curses herself because she is the princess of love she shouldn't have been THAT oblivious. So they decide to give a romantic relationship a try.
2) Most people know Fleur for her modeling career, and not because she is a Knight, in fact she graduated as one along Shining Armor! He knows she isn't a dainty little dove, in fact she was one of the best members of the academy.
both of their spouses LOVE seeing their sweethearts in those fancy armors.