There's really no reason to multitarget Silverlight. Unlike CLR, there isn't a side-by-side story: people run the latest version.
There's still an argument to develop for Silverlight 3, so go ahead and use the Silverlight 3 development tools - your Silverlight 2 apps should work just fine.
There may be some extremely limited, controlled environments without Internet or IT connectivity with Silverlight 2, but I'd expect that to be vary rare in my opinion. In such a case you could use MSBuild to build Silverlight 2 (even if you have newer bits on your machine) by placing the build tasks in your enlistment and redirecting from the standard SDK: http://www.jeff.wilcox.name/2009/03/sxs-sl2-sl3-building/
Here are the Silverlight 2 tools for Visual Studio 2008 SP1. You won't be able to developer Silverlight 2 apps in VS2010: http://www.microsoft.com/downloads/details.aspx?FamilyId=c22d6a7b-546f-4407-8ef6-d60c8ee221ed&displaylang=en