Two Safari's at once? The goal is to have two (or more) versions of Safari accessible on my mac at the same time. I know this is possible because I have done it with iMovie 9. I now can run iMovie 9 and the latest version of iMovie at the same time. I need the older version of Safari for web development.

I think the only way for me to do this is to copy it from another older mac or find it online. To have an older version or newer version of an app, just select the app and do command-c and to paste it do command-v. To prevent your copy from being updated, copy it to an external drive.

If someone is still running a version of OS X between 10.7 and 10.10, you should have the versions of Safari I'm looking for. We can collaborate and do something on Google Drive to move the app across to my mac.

Strange question but I think that this question could benefit a lot of people especially those who do web development such as myself.

I currently have the latest version of Safari (9.0.1) on OS X El Capitan.

Since Apple only offers the latest version of Safari to download, the only way to get an old version to a 'new' computer is to copy it over.
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...