Becky,
        Yes breasts frequently (usually?) produce differing amounts.
Sometimes substantially different.  On two occasions I have seen women who
have had NO gland development in one breast and have successfully fed their
infants from the other breast.  The mom's situation sounds like a common
one, just complicated with the twins.  Obviously this will take some
management.  If she chooses to feed both twins at the same time, she may
need to 'top up' the twin who has fed on the lower producing breast by
switching it onto the other breast at the end of the feed. In some
situations I have seen, the mother has used a supplementer (SNS, SUPPLY
Line, LACT-AID, etc) just for the twin nursing on the lesser producing side.
However, before deciding that the unevenness is permanent I would have some
questions.  Was the mom feeding the babies consistantly from the same
breast.  IE baby A got the right side at each feeding, baby B got the left.
In that case the uneven stimulation may have caused or exaserbated the
problem.  Ensuring she switched sides would help. If she is pumping
regularily, is she seeing a response on the lesser side?  Is the volume of
milk increasing or remaining stable?  Does it appear that the larger
producing side will be able to increase production enough to maintain the
twins -- assuming it is not already doing so.
        It sounds as though your assistance has already been of help. Good Luck.

        Rhoda