I use Lydia as Steward for Lakeview Manor, but I also have Rayya at the same time.
I used Rayya from Falkreath as follower before Lydia was asigned to me in Whiterun. By this time I hadn't decided whom I'd chose for Steward and told Rayya I would think about her offer to be steward and played onwards.
I like Lydia from several games. She does a good job, is polite and correct. When she was given to me it just came natural to me to give her the steward job. I dismissed Rayya and had Lydia accompany me. When we arrived at Lakeview she asked about the job and I gave it to her. End of story.
Right after this I went back to get Rayya as a follower again. We took some quests together.
At the time I joined the Companions I released her. I knew from a previous game she couldn't follow me anyway. She replied she would go back home and 'do some rounds at the manor'. Fine by me.
After a couple og quests with the Companions I needed to get home to make some better armor and stash my stuff. I met Rayya circulating the estate as a guard.
Huh? Ah well, she told me she would, so fine by me. But now she also sleeps in my house!
I got both Lydia and Rayya as Housecarls.
Maybe the game lets this happen because Lydia is officially asigned and Rayya originally belong there?
Quite nice with two Housecarls though.