Toddlers & Stairs...

Both our kids slept in our room until they were about 16 months. We just slept better knowing they were there with us. We put the crib in our room, though truthfully they slept in our bed more than the crib. We have a two bedroom house, and our boys have been sleeping in the same room since the youngest was about 16 or 17 months, right about the time our oldest turned 3.

Many, many kids grow up sharing a room at a very early age. I would have no reservations about that. I would definitely choose that option over having bedrooms on different floors (obviously, I'm the one who had a newborn in my room because across the hall was too far away :-). When they get older, it might really be nice for them each to have their own room upstairs. I've heard lots of people say that when their kids were 8 or older. We have an extra bedroom in the basement, so you never know what the future holds for us.

I don't know how old your toddler is, but my kids mastered the stairs by 12mos. It was just part of their lives (we live in a 4 floor townhome). They learned how to slide down on their bellies, and eventually walk down holdin the railing just like they learned to crawl and walk.

So before you make any changes to how you set up the bedrooms, why not just get your first comfortable on the stairs...and you comfortable with him? Invest in a good baby gate (may I suggest Retract A Gate?), and just close it off when you need to keep him in one space.
I would move his room upstairs, so you're not climbing stairs for the baby in the middle of the night. You can gate the stairs at night so your son doesn't come out and fall down groggily. You'll be able to hear him if he's up and needs you.

Whatever you decide, good gates -- that fit well and that your toddler cannot operate on his own but needs you to operate -- are vital. Shop around. Do not ever, ever use "pressure gates" for stairs (the gates that just slot into a doorway and are held by tension but have no hardward bolting them to the door frame. Look on these and you'll see they are not approved for use on stairs because just the weight of a kid leaning on them can pop them out of the doorframe and kid and gate will both go down the stairs). The best gates cost, but are worth it.

Personally I would not be comfortable with a toddler sleeping on a floor of the house where there was no adult, in case of many things -- wandering at night, nightmares, not being able to hear what the child was doing in his room, etc. And remember, the toddler's sleep is going to be disturbed when the new baby comes, not just by the baby's noises but by your son's restlessness and the upset to his world when the baby arrives, so he may be up more at night too and you want to be where you can get him back to bed quickly and not be going up and down stairs yourself.

I guess they could share at some point but your house layout would be a tough one for me in your situation.

Your older son should be fine upstairs, just use a gate at top of stairs and a baby monitor to help you hear him. He'll be at least three ish before the new baby comes, right?