Just another voice of reason saying, "will have to" supplement is a bunch of bunk!
My twins are now 13.5-months-old. They still nurse. I've been back to work full-time for 3 weeks. It's all good.
Here's a snap-shot of my nursing history:
Nursed the three older singletons for a year each with no supply issues. All were 9-11lb at birth and grew very quickly 90th-percentile and over.
Birthed healthy, happy, full-term twins and assumed that while they were smaller (ie. average: 7.5 and 6lb15oz) that they would be "racing out of the blocks" and grow very quickly after birth - catching up with their singleton siblings (as we assumed was their genetic predisposition). BIG, BAD, MISTAKE!
This led to us becoming completely perplexed that our content, perfect little twins should be examined by public health nurse and lactation consultant at 2.5 weeks old. They had slow/stalled weight gain for the first month-ish. We just couldn't understand why they seemed awake enough, sleepy enough, content, healthy, proper diapers, etc, etc but weren't growing bigger. REPEAT: EVERY CHILD IS DIFFERENT, every child is different (even if your other 3 have been the same
Then we were in it: a hoopla of nursing, weighing, pumping and supplementing expressed breast milk, ya-da, ya-da, ya-da. It wasn't until I was told at 7 weeks to go on Domperidone (by the lactation consultant, who had previously agreed that there was no supply issue) that we put our feet down and jumped off the "silly train". In retrospect, it was just self-imposed insanity. . . and who needs that when you've just had twins!
The public health nurse was very pushy about supplementing with formula, saying we wouldn't "have to" do it for long and that she didn't "need it" for very long with her own twins. Too quick to assume that "did it" was equal to "needed to do it". It wasn't until we finally were able to see a family doctor we quite like, when the boys were 2.5-months-old, that he said "Some kids just won't follow the growth chart. End of story. As long as there aren't other
concerns in concert with the lack of weight gain, it's a non-issue".
The whole ridiculous matter resulted in us weighing our babies like crazy over the first year to document their growth as information for other twin parents. They started out 1/2lb apart. This got as wide as 1.5lb apart. Now at 13.5 months, they are 200gr apart (7oz). As we've gotten to know them, it has become obvious that this difference is personality-based. The squirmier, more sociable one is lighter - no time to eat, Mom!
I think the idea of parenting twins seems so burdensome and overwhelming to many other people that they just can't imagine you doing it successfully without their "help". Good-hearted or not, sometimes it's just a bunch of hooey. You'll know the difference. You can do it. You will