Here is a link to another thread on this same topic: "Two Mommies BF-ing?"
My dw was not interested in breastfeeding our kiddos. She did let our twins latch on a couple times as babies, and a few more times as toddlers, but she didn't like the way it felt.
I think it is a really cool idea.
I would urge anyone considering sharing the breastfeeding to be cautious, and to work hard to maintain the birth mother's full supply. I know one couple who decided that they would both breastfeed their twins--two moms, two babies--makes sense, right? The non-birth mom induced laction before the babies were born, and her milk came in wonderfully. They both shared the nursing of the babies pretty much right from the start, and couldn't imagine how any one woman could breastfeed twins. About six weeks later, the non-birth mom went back to work, and the birth-mom no longer had enough of a supply to breastfeed her babies on her own. They ended up giving up and switching to formula.
A non-birth mom can also comfort nurse the baby without inducing lactation. We have friends who have used an SNS or lactaid (supplemental nursing devices) with expressed breast milk from the birth mom. They use it the same way some moms use an occassional bottle of expressed milk, only the baby sucked on the non-birth mom's breasts instead of a bottle nipple.