Can I install OSX Safari 5.1 for one user and Safari 5.05 for another user - same CPU?
The person behind Multi-Safari has documented the procedure to create a Safari application with a self-contained WebKit framework here.

Basically it involves duplicating the Safari application, opening the contents and creating a Frameworks folder inside, copying the Framework from /System/Library/Frameworks and then performing some jiggery-pokery to get the application to prefer to use the self-contained framework over the one in the Library folder.

(I decided not to copy his instructions word for word, as his website didn't use Creative Commons.)