I don’t think you can have two Mac OS’s on the boot volume. Boot Camp just lets you have an “alien” OS (Windows) on the same volume you boot from. You need a separate partition on your internal drive (or a second hard drive) to use for your Snow Leopard system, along with the apps you ordinarliy use. This may mean some re-installing on the alternative boot volume, or some restoring to it from a Time Machine back up. Since I remain wedded to Snow Leopard I have had no experience doing this, but the literature says it is possible.