Should work as long as they are in separate vaults.
After a little investigation, the Add-In(s) are installed into separate areas for each vault. However, the add-in loaded corresponds to the first vault view entered into via Windows Explorer. Once the DLL is loaded into memory it is not unloaded & reloaded as you change vault views. So, you'll just have to be a little careful. Should work fine for my needs.
As for Task Add-Ins, I didn't test it yet, but I suspect each time TaskExecutor.exe starts up, it will load the corresponding add-in for the vault.
EPDM 2013 SP3
Is this a .NET or c++ add-in? Not sure how c++ add-ins are loaded but you may have issues with .NET ones. Once you access the add-ins (regardless of the vault) - the dlls will be loaded into the explorer.exe process. Now if you are using the same dlls .NET will load only the version which was accessed first. So let's say your add-in is using the A.dll and it was modified in the second version of the add-in. In this scenario both of your add-ins will be using only one version of the A.dll (whatever loaded first). You might fix this issue by using binding redirect: <bindingRedirect> Element