The Windows Phone emulator only supports multitouch gestures with a multitouch device. You would have to have a screen, or touchpad to work with it. Also, there has been comments about using two mice and there is a project on codeplex http://multitouchvista.codeplex.com for it.
A short video demonstration about using MultiTouchVista to emulate a pinch gesture on Windows Phone can be found here: http://vimeo.com/10330829