I remember discussing this issue with Duchess Deshive when I lived in Caid. She said she would never dream of sharing the throne with someone she was actually romantically (and/or maritally) involved with; there would be no escape from it.

I was at the first Decoronation of Jafar (upon whom the peace and the blessing) and Katherine of the Midrealm. The visibly-pregnant Katherine called up her husband to give him some kind of award, pointing out that "not since the days of Arthur and Guinevere has any Queen been so well served by her Champion...."