Many people are forced to use multiple source code control systems on a single PC. For example, I do a lot of side projects that are stored in CVS. I have another project that has some VB code stored in PerForce. And I'm working on a new project that will likely be using SourceGear's Vault product.
On top of those three systems, I'm a zealot on Source Control and try just about every product that comes out. This means that I often have 5-10 different systems installed at the same time.
For some reason, Visual Studio only supports a single SCC provider called the "default" provider. This always struck me as odd because it implies that there can be "non-default" providers. Well, there can, you just can't use them. :--)
The fix has always been to edit the registry. Or if you are like me you would create a bunch of .reg files to make the edits a bit quicker. This works but is a real pain in the butt.