Maybe it refers to one's milk mother and birth mother.
Some people have two mothers!
Like a man marrying more than one wife, such as Walder Frey?
Well, I was thinking in a person adopted by lesbians...but yep, I guess that your example count as one XD.
Yes, I also think of lesbians case, but I'd like to map it to ASOIAF context.
Targaryen kings commonly married more than one women (preferably their sisters). An heir might consider all the King's wives leaders and mothers.
Well there's Yara Greyjoy. Lol. However, I don't think it's allowed under the Faith of the Seven.
It could also be a great come back if the enemy says your army is full of mothers and children, suggesting they are somehow weak. You'd be saying, yeah but those mothers can kick your ass and order your death.
Muña not only means "mother", but also "mother's sister" (i.o.w. a specific "aunt").