It works like this..you get 2 rings.
you can use whatever toon you want to "marry" with whatever friend's toon you want.
That means, you can either play together with 2 different people ,or the same guy with 2 different toons.
Once you performed the ritual the Rings are engraved, so they only work with that specific character.
And don't forget you have to wear the Ring to get the Bonus.
There is no Divorce in ESO