[DiapDealer]

Here's a sample batch file to get someone started. Just remember that you'd always have to start the alternate installation by running/clicking the batch file. If you double-click an epub, or right-click on one and use Open-with Sigil, the environment variable won't be set.

You'd have to launch Sigil with the batch file and then use File->Open.

You can even create a shortcut to the batch file, change the icon to the white Sigil icon in Sigil's "Misc" folder (or whatever icon you want) and name it anything you like.