I went through the same thing last December. I was really worried that they would never sleep, but turned out to be much easier than I thought it would be. The only part I regret is that I think my 4-year-old gave up naps earlier than she otherwise would have. They just couldn't fall asleep in the same room at nap time. I wish I'd have considered having her nap in my room, but it did not occur to me. The 3-year-old still naps well.
Night-time can be difficult, but I've found that, having given up naps, the 4-year-old usually falls alseep really well. My 3-year-old sometimes has a harder time, but my 4-year-old tends to sleep through his calling-out for me. In fact, I've been completely shocked at how much each of them sleeps through when the other wakes in the middle of the night for potty, water, or anything. Yes, they definitely give me trouble sometimes, but usually after a couple threats of toys getting taken away for the day the next morning, I can get them to calm down. I just want to tell you that I wish I'd have know that it wasn't going to be as bad as I was worried it would be. I wasted a lot of time worrying about it. And they LOVE sharing a room and really seem to sleep better knowing the other is right there. I wouldn't change it now even if I had an extra bedroom appear in the house. Of course now we're thinking of a 4th, and I'm terrified that it's going to be much harder!! We'll see! And I'll definitely be checking your response list to see if anyone has had much younger babies share rooms.
Congratulations on your pregnancy, and good luck to you.