you have to eat an extra 500 calories for the milk supply and an extra 300 calories for the baby. you actually lose the 500 calories you ate through breastfeeding. and normally you dont have to actually start eating the extra 300 calories till your baby is about 25wks or so. so right now just focus on a 2500 calories diet and you be fine. until about 20weeks you need to be on a 2800 calorie diet. of course youll lose the 500 due to breastfeeding, but the extra 300 will be for your baby. youll be fine =)
but of course if you dont keep up w all the #'s youll just end up being verrrryy tired because both babies will take up all the nutrients they need =)