dr. jack, this is not so unusual as it may seem to you. try, lesbian
grandmothers wanting to breastfeed, etc. but, look at this situation as
analogous to adoptive nursing, not father feeding the baby. you should
encourage the other mother to nurse for all the same reasons we encourage the
adoptive mother to do so, and not base it on whether she produces milk, or if
there is other breastmilk available. or, it could be regarded as the same
question as, "should my sister, close friend, etc., nurse my baby when
convenient?"

(some years back a book came out for children entitled "heather has two
mommies," which was in our then communal-living household. i  found it to be
rather funny, because heather's two mommies acted out the gender stereotypes
of male and female so uncannily. others from the right and the left have not
found it as humorous as i, but then, i am infamous for my sick sense of
humor).
(nursing is "having chi-chis" at our house)

carol b.