Second screen mirror one desktop While I am using multiple desktops on my Mac, I would like to mirror one of these desktops on a second monitor. 

This monitor would keep mirroring this desktop while I could switch between all of my desktops with my main monitor.

Any ideas on how to make this happen?

I am using an iPad as a second monitor if that might be important.
Welcome to Ask Different!

I am assuming you are using an app such as Duet Display or Air Display for this purpose. It sounds like you are attempting to extend your desktop, rather than mirror a desktop. You will need to go to System Preferences -> Display -> Arrangement. From there, you can shoes to extend the Desktop. 

Following these steps, you will be able to have one or more windows open on your iPad screen, and several desktops on your Mac screen as well.