Is 15 - 18 months too young to move to a toddler bed? With DS1 we bedshared until he was about 5-6 months and then moved him over to his crib. We're hoping to do that with #2 and then move #1 to a toddler bed and #2 to the crib. Is that realistic?
If we have to we'll get another crib but I don't want to buy a second just to buy a toddler bed a few months later.

Re: too young for toddler bed?
I hope others post with their actual experiences.
We put DD in a toddler bed when she was 15 months old and she did great! That being said, I don't think DS will be ready at that same time. I actually don't think he'll be ready until he's closer to 2, maybe a bit later, especially since he'll be sharing a room with the new baby.
You could always try it out by putting the crib mattress on the floor and seeing how he does and at least then you'll know.
DD is 21 months old and is not even close to climbing out of her crib, thus she is in her crib still.
DD and DS are 14 months apart.
I am so glad we got a second crib for DS since it was nice to have somewhere to put DD if necessary (like if I needed to shower) to keep her safe.
Some people do move that early, but I'm waiting as long as possible.
I'd wait until LO #2 is here. I almost bought a crib ahead of time, but I'm glad we didn't. We moved LO #1 to a twin bed around 17 months (we skipped the toddler bed because we were bedsharing previously and wanted to be able to lay in bed with him if he needed extra cuddle time).
See how your son is at that age. If it's a relatively painless transition, move him and save the money. If not, you can always buy another crib then. I've always heard that it's more convenient to have them confined to a crib, but I dont get that mentality. We keep a baby gate on his door so he cant wander throughout the house, and his room is completely safe. The worst he's done is opened a drawer and threw some clothes on the floor from having the freedom to get out of bed when he wants.
GL.