You can see an archive of older versions of DEX at https://dexpcb.com/Download/Archive
DEX Release Archive
You can install more than one version of DEX on the same machine. They will share a common set of configuration files. However you will need to disable auto-updating otherwise whenever you run an old version it will prompt you to download the latest. You can disable auto-updating when DEX asks you if you wish to auto-update, see dialog below:
Click the cancel button to cancel auto-updating. DEX will then ask you if you wish to check again next time the program runs. Click the No button to disable auto-updatings.
To re-enable auto-update click on the Get Latest Version button 
in the Home→Account menu