Greg,
You should be able to use two different sites within the
Exchange implementation, though if you want to test a more robust
separation of components, you might consider running Exchange in two
different virtual machines, using VMWare or Windows Virtual PC or the
like. It's amazing how much flexibility today's virtualization tools can
give you.
A lot of us in IB dev use machine images for Windows Virtual PC 2007; it runs great in Windows 7, has backwards compatibility with images created before we upgraded, and doesn't seem to use a lot of extra resources. You can still install this version (http://www.microsoft.com/windows/virtual-pc/support/virtual-pc-2007.aspx), though if you're just starting with virtualization, you could also install a newer version.
--Robert