Two Safari's at once?
You can run a semi-autonomous second instance by downloading and installing the webkit nightly build. You could run an older build if you're not keen to experience the bleeding edge bugs that sometimes creep into the builds. 


http://nightly.webkit.org


It's not truly a second safari, but you don't get your hands dirty worrying about two instances of the app trying to read / write to the same files for history, bookmarks, etc...