Houses were originally designed for FCs so tenancy was built in from the start. It became useful when the demand for houses exceeded the supply, allowing players to share a house.

Apartments were designed for individual characters so they probably didn't see a need for tenancy. Any character that wants an apartment can go buy it so there's no need to share to gain access to one.

Is there something specific in the programming that prevents tenancy from being possible, or is it just a design choice? Only SE knows.

My suggestion is for you and your fiance to have apartments in the same building so you can switch between them together as if you were moving through a door into another room. That's what a friend and I are doing with our alts, and it works out well.