I nursed my singleton during my twin pregnancy, with the full knowledge and support of my perinatologist (our boys had TTTS). My milk lasted through the first trimester, but not very long or maybe a little before the second it went away/slowed waaaaay down. (It's been awhile so I don't remember exactly at what point). It wasn't a super big deal to my toddler, she was 17 months old when they were born, so she had been nursing quite frequently but was eating some table food by the time my supply vanished. Once my milk came in from the boys she picked back up again (thank god, NO ENGORGEMENT, best thing ever is a nursing toddler). It wasn't a big deal. I did have to chase a lot of LCs away though and at that time no one in the local LLL group had much of any kind of resource to offer. (This was about 11 years ago). I think there are more resources now, but I winged it and it wasn't a big deal.
I think you just deal with problems as they come up. The TTTS wasn't a problem, but if I'd had to have had the surgery or if I had any kind of preterm labor issues or whatever probably things would have been different. So you just roll with it, shrug off whatever ignorant comments you get but keep an open mind for things to watch out for from a provider you trust, I bet it will be just fine.