I've run into this too. It's tough and tricky separating the biology from
the psychology of the situation. The biology is the easy part.

My personal thoughts are that nature designed ONE mom per baby, and
sometimes one or more "others" who are important to the baby. But every
baby only has only ONE biological mom. Competing for the baby isn't good
when it's a dad or grandma or nurse, and I'd worry that the "other" mom
could get into competing for the baby.

There's no one right answer here - I try to identify the questions and
issues for the single-gender family. For all families, for that matter.

Linda J. Smith, BSE, FACCE, IBCLC
Bright Future Lactation Resource Centre
Dayton, OH USA
http://www.bflrc.com