Assign Application Windows to Different Spaces/Desktops
You might make a copy of Safari's application bundle and change the bundle identifier:

cp -R /Applications/Safari.app /Applications/Safari2.app
defaults write /Applications/Safari2.app/Contents/Info.plist CFBundleIdentifier my.safari2
cp ~/Library/Preferences/com.apple.Safari.plist ~/Library/Preferences/my.safari2.plist


Assignments to spaces are per bundle identifier:

$ defaults read ~/Library/Preferences/com.apple.spaces.plist app-bindings
{
    "my.safari2" = "FC886111-EE78-4844-A5E5-A11BECA713CF";
}


In this case, you might also use Webkit.app or create a site-specific browser with Fluid.