Koukyuu Kisaki no Kanrinin

Yuuran, the daughter of a major merchant, was hurriedly married off because of an imperial edict, in order to manage the imperial harem. When her husband, the younger, handsome right prime minister came, she could only apologize to him for having to marry someone as old as her.

But when she went to do her duty at the Imperial Palace, a beautiful woman appeared, and said "I do apologize for showing you such an unsightly appearance. I am your husband."!? Entering the harem as a couple with a cross-dressing husband!?

(Source: Kadokawa, translated)