Hi, and welcome!
My daughter was 17 months old when my boys were born. They had TTTS (shared placenta, different sacs). I'll address the parts of your post that I'm most familiar with.
First, TTTS is rare, even amongst MZ twins. HOWEVER, it does help if you have a practitioner that is not totally ignorant of TTTS concerns, and if you were told that two separate sacs means you don't have to worry about TTTS then whoever told you that is ignorant. Because TTTS is so rare, many doctors don't know what to do or what to look for. The things to keep an eye out for are size discrepancy and ambiotic fluid discrepancy. If that starts to happen, you will probably need a specialist AND to contact the TTTS foundation who will be able to educate both you and your OB. But...that is putting the cart before the horse. The odds are in your favor that you will only have to worry about this because we mommies worry over everything but you'll never have to deal with it.
Second, NO you do not have to wean. I nursed my daughter through my whole high risk pregnancy with the approval (not that I asked for it, but it was nice to get a thumbs up) from both my perinatologist AND my OB. You may start losing your milk soon and at some point it will switch to colostrum. I think I only truly lost my milk for a couple of weeks--everyone's different though. Let me tell you though, if you can at all swing it you DEFINITELY want a nursing toddler after birth!!! Twin engorgement is spectacular when it starts to happen but your eager toddler will be happy to take care of it for you so you can pump if you'd like or get the babies on the breast without having rock hard boob problems. Of course, if you start having productive contractions due to nursing or something like that, you'll need to wean, but...as I said everyone's different, I didn't remotely have a problem with that, so I got to tell OBs and peris other than mine to (nicely or not, depending on how pushy they were) to bugger off if they decided they'd like to comment on it.
Third--I got a triple stroller. I hated it, but I hated the alternatives more. CHeck with mother of twins clubs to see if someone will sell you their old one (or chuck it at you to get it out of their house) BEFORE you buy a new one. (Do you live in the Seattle area? I have one you could have for free if you do.
) if you decide to go the triple route. But I do know folks that have done a sling/carrier for one and put the other two in a double (rotating which baby is in the carrier). I liked the skinny strollers (the kids are behind each other, rather than being side by side) best since I had to take the kids shopping a lot in it and it was easier to negotiate aisles. But I know people that swear by side by side too.
I'm never one to downplay how scary this can all be. And I won't lie--you'll likely have to endure having to walk the thin line between smart use of monitoring and beating back people who want to intensely medically manage you. Multiples moms need to be tough--if you're not, don't worry, you'll get there faster than you think.
Don't let doctors or the Oh-My-Kids-Are-Just-Like-Twins-Let-Me-Tell-You-How-It-Is people bully you. You still get to make the decisions, and you can still be an informed, confident mama. You just get to do a little more reading, is all.