If you want to test a skin, it is best to have it offline, using xamp or a similar program which creates a virtual server on your pc. You can download a copy of your db, and your phpvms installation, and test it out on that before putting it live. You can't set it manually in the layout.tpl as it would have to be changed everytime you want to change the site design. And that is just tedious. But as servatas said, just use the same folder name for both skins, but have a different layout in the one of them, that means you can have two different layouts and designs with the same db.