Can Spaces be used on only one monitor (with another displaying just one desktop)?
I don't think it can be done...  However, you can assign applications to be present in every space.  So, if you end goal is to be able to have your e-mail always showing, yet use spaces to manage other applications, you could do the following.


Set mail.app (or whatever mail program you are using) to be present in every space.  Then drag the windows to your second monitor.
Launch several other applications, each in their own space.


Now you can switch from space to space and have Mail available in all of them.

Not exactly what you asked for, but I suspect that it will serve your purposes.