Ask your hubby what the poops look like, color and consistency. They should be transitioning from that black tar to quite liquidy and yellow. In between, they will look green, brown ,etc with varying consistencies. Any formula changes the way poops look (and smell.)
Obviously, feed the babies. If you have to supplement, do it. But make sure you REALLY have to. Pumping, supplementing, and around the clock nursing is way more exhausting than just around the clock nursing. 10% weight loss is scary but only slightly more than normal-most babies lose weight after birth, especially a birth with lots of interventions.
How much help do you have? You need more than just the other parent when it is twins. REAL help, live in if possible, who will support you and take care of you so you can do nothing else except heal your body and feed and snuggle your babies.
I had a c-section with my oldest who came at 37 weeks after a traumatic labor and my milk "came in" on day 6. So this is little late but can be normal. But remember this:
1) Colustrum (the first milk that comes right after birth) is super sonic food, is very, very scant, sort of a honey color and hard to see, (also hard to pump) and very very important for babies to have, and they get it my nursing a ton in the early days. (Nursing a ton also brings your abundant milk in more quickly). Colustrum IS milk. And how. Super sonic milk. Just not the abundant white looking kind. And babies do not gain and usually lose some weight in the early colustrum days. This is normal.
2) SOME moms have leaking, engorgement, giant inflatable breasts etc when milk becomes abundant. But many do NOT. If your babies are nursing efficiently and often, this will help prevent any engorgement, which is actually something you do not want to experience if you can help it.
Look at weight gain and output (poops) to be sure your babies are getting enough. Make sure all weight gain is measured from the lowest known weights, not birth weight. make sure weight checks are done accurately and on the same scale every time. Nurse lots and lots. You cannot nurse too often.
Book suggestions: Mothering Multiples and The Womanly Art of Breastfeeding (8th edition)
websites: http://www.karengromada.com/ & http://www.biologicalnurturing.com/
articles: handy diaper log: http://www.llli.org/docs/00000000000...diaper_log.pdf
feeding cues: http://www.llli.org/docs/00000000000...eding_cues.pdf
waking sleepy baby (if needed) http://www.llli.org/docs/00000000000...py_newborn.pdf
Great article on what is expected adn normal in the early days: http://kellymom.com/bf/normal/newborn-nursing/