My twins are 3 months old now. In the beginning, I let both of them to latch on, one by one or both at the same time. After 1 month of tiredness and because my boy drinks very slow, I started to express the milk for him to drink while totally breastfeed my girl. The volumn of my express milk was also not high. I have to give him FM as well. But after I find out that it is quicker to get the breast milk while my girl is latching on because of the let-down effect, my situation improves, from 2-3 FM per day to 1-2 FM per day.
I also wonder whether my girl drinks enough or not because she gains weight slow. But the doctor said she is ok and she doens't cry for milk, I think it should be alright.
Why do you need to express for 1 of the babies? I feel it is more comfortable to have them latch on because the pumping machine makes my nipples a bit painful.