I can't seem to find any related searches to this so thought it may be useful to some people to read about my experience. I'm not sure if someone else has tried this before, wouldn't surprise me if they had done.
I'm running 0.8.5.1356 (or rather .1333 until recently) with a non-facebook linked account; a fan of the older less bloated and social versions of the client. I stopped the automatic updates a while ago and have been relatively pleased with how everything has been running. I had some issues with playlists appearing in searches, and I thought what if my older client had something to do with it.
This got me thinking could I simply have 2 versions of the Spotify install on my mac, using the 8.5.1356 version and the newest install to try out without losing the old client and faffing around with re-installs and syncing. Essentially I think you can however I've run into a few problems.
First you need to download a version of Spotify, and make sure you edit the archive sig file in the library (good instructions available on the forum) to stop auto-updates. Then rename this version of Spotify to whatever you like, I simply added the version number. Next download another version of Spotify, and drag this to your applications folder, renaming it too to something you like. I had 8.5.1333, then downloaded 8.5.1356, and 9.7.16.
I didn't experience any issues until I had opened each version (so 3 versions in total), which combined with my phone, made 4 instances of Spotify. I believe may have triggered a removal of offline access to my Spotify account as I have gone over 3 registered installs. As when I opened my now favourered old install, offline access had gone and I had to resync all my stored tracks.
Also, the location of stored music and associated settings kept resetting itself. If you tried to adjust these settings, the client would crash, unless you selected OFFLINE mode.
I think I need Spotify HQ to remove my duplicate accounts or excessive registered installs, so I'm just left with registered runnings of 8.5.1356, 9.7.16, and my IOS install 7.2, which all have offline access. This way I should stay in the limit of 3 Spotify instances, and I'm guessing now, not revoke offline storage and have to constantly resync.