In this blog post, Mihail Mateev discusses the simulation of the Multi-Touch activity for the WP7 applications.
Source: Mihail Mateev's Blog
Currently, most computers do not have Multi-Touch Screens.
One very easy option is support for multiple mice. For this purpose you can find different solutions as Multimouse for Windows 7 or Multi-Touch Vista in codeplex. This article uses Multi-Touch Vista which is free and quite popular.