We pretty much had it drilled in our heads during LC training that at least 2-3 decent sized poops a day for newborns (the first 4 weeks) was necessary to tell that the baby is getting enough hindmilk. Lack of poop may be due to not enough hindmilk.
I honestly would see a Private Practice IBCLC and have her do a few pre and post feed weighs. Urine output is OK for older babies, who can go a few days without stooling and then have a huge blow out, but newborns really should be having 2-3 or more stools a day of at least the size of the palm of your hand.
The hardest part about tandem nursing is making sure the newborn gets both enough foremilk (high in lactose for brain growth and antibodies) and hindmilk (high in fat for growth and satiation.
Your baby may well just be a reluctant pooper. Or, if his one or two poops a day are really huge (like blow out diaper huge) he may be of course, getting enough of everything he needs. Does he stain the diaper with most feedings? How big are the poops?
I'd advice a visit from an IBCLC who will do pre and post feed weighs and will check his mouth and the latch, just to be on the safe side. It's easier to deal with any problems now than to try to play catch up later.
Edited by MaggieLC - 10/26/13 at 10:55am