My boys are almost the same age and I keep wondering when would be a good time to put them together. TBH, I have no idea how it might go, but I think that they would just get used to each other!
Maybe one weekend you can try a 'sleepover' in one of their rooms, to see how it goes? Do you have a pack n play that you can put in your older child's room for the 1yo to try sleeping in?
I would definitely try to make the transition soon so that they have lots of time to get used to it before the baby comes Maybe spend some time talking to them about it, especially the older child. And also make it a special space for both of them, and keep their bedtime routines consistent while doing the transition.