if you have a big enough bed, you can have one baby on each side of you. Just get your 2-year old used to having your back to him now, not later!
But if you want to transition him, slow steps are best. Start now, with his mattress or a sleeping bag on the floor next to or near your bed. Move closer to door gradually, and just outside, etc. Use rewards as incentives. Be positive, never punish for steps back. It's *all* progress, it really is.
Hopefully you have a little while yet before your baby is due. It might be a few weeks or a month or more before your son is ready for his own bed, and a big change like this is likely to slow down the process a big. Be ready for more time with your son in your bed when the baby arrives! Probably a mix of these two approaches will be the least stressful for everyone involved. At least if you can get your son used to sleeping *near* you, not right next to you, this will be a great help when baby is among you. Of course the sound of the baby crying will probably also wake up your son, but he might prefer to be near you, anyway.
For more information, read the Sears Nighttime Parenting Book. A nice, quick read :) Also, there's the No-Cry Sleep Solution by Pantley. Both deal with nap questions, too.
Safety tip for sleeping with both babies in the bed: have a pillow between your husband and your 2yo.
Have fun! Good luck!