Can I use two wireless keyboards with my iMac?
You can connect a wired and wireless keyboard at the same time.

I don't have a way to test two wireless right now, but how are you doing it?  Just turning on the second keyboard won't work. Your iMac won't bother because one is already connected. Have you tried connecting manually? In the Bluetooth preferences you can manually connect to other devices. Of course both keyboards have to be paired first. If you have the Bluetooth menu turned on in the preferences, you can try connecting from there as well.