In the etymology of the word. "Mom" is derived from "mamma" which is a root for "mammary" and for "mammals", meaning those animals groups in which the females provide milk for their young.
The very meaning of the word generally connotes that there is only one "Mom." Unless both women are suckling the babies, both of them cannot be "mom." If both are suckling the babes, then the proper conjugate is "Mams".
“If both are suckling the babes, then the proper conjugate is “Mams”.”
That may be the case! The one had the eggs taken out of her and put in the other gal. Then a couple years later vice-versa! Really weird on one hand - but I thought that was an interesting way to do it. (Less of a “this one is mine” type thing.)