Yes, that is what I mentioned in my original post. But, I believe that will only switch a library, not start MC with a different library if one is already running. I want to keep my mp3 library running minimized at all times but be able to open my default library at times without interfering with the mp3 library that is serving my Roku.
Sorry if I didn't explain well.
You can run multiple instances of Media Center:
Options > General > Advanced > Allow multiple instances to run at one time
Running MC12.exe talks to the existing instance, and only starts an instance if one isn't running.
Running Media Center 12.exe starts a new instance.
However, you might consider a way to use smartlists or similar to avoid multiple instances. Multiple instances may cause other problems like running two UPnP servers, etc.