Someone recently pointed out that the mother dog, cat, mouse, with multiple nurslings, doesn't worry about which breast which baby is on.
The point being that with six nipples (or so), and six nurslings, it doesn't matter to the mom which one is where.  (Though individual babies might bond/imprint on particular boobs.   All the babies are getting their food, all the teats are being emptied.    If human moms always had at least two infants, we woudn't have to be concerned, either, if a baby developed a one-sided preference.  As long as both twins didn't want only the same breast.  Even so, mom could technically produce enough milk -- just how comfortable would she be?

Just met an interesting mom/situation.  It is her second baby, two days old.  First infant successfully and exclusively breastfed for a year, plus.
on only her right breast.  Developed a preference at birth, and despite much encouragement and many offers over the days, months, year, never took the left breast.  (And yes, she did have studies and mammography done to discover if anything suspicious or negative  was going on.  There wasn't.)   Second baby is now repeating the pattern of the first.
Mom's attitude is very relaxed -- she says she'll just do the same thing.
Nurse the baby on the right side, ad lib.  And pump the left, usually at the same time, to have milk in storage for while she is at work, out of an evening, etc., etc.  

Those with animal experience:

If you watch a particular litter, do they trade off who sucks where?

If an animal only has four pups, kittens, whatever, do only four nipples lactate?  Or do they all develop milk, but then the unused ones drop back on production?

Thanks.

Chanita, San Francisco