Answering my own question.
So Rhino knows which rhp to load after you install the rhp in the plugin manager. I forgot that the first time we debug our plugin, we need to install the rhp by telling rhino where the rhp is. All I have to do for my duplicated VS solution is to reinstall the plugin by specifying the path of the rhp files.
Here is a link that will help, especially the bottom of the page:
http://wiki.mcneel.com/developer/rhinocommonsamples/dotnetexpresseditions
Cheers,
Yifan