[thejeff]

As far as I know, as long as you're playing on the same machine (IP Address? MAC Address?) you can use the same install (CD-Key).
I've seen claims otherwise, but I don't recall anyone posting that they tried it and it didn't work.

I haven't tried this. It obviously would work in hotseat games. And I'm not sure how to experiment on my own. I've only got 1 key, and I'm not sure that running the experiment while playing from the server machine would be a good test. I could set up a test 2 nation game if you wanted to try it out...

I'd really hope it's that way. It's certainly legit for copy protection to prevent using multiple installs of the same license, but to prevent using the same install to play 2 nations is not. No copying, no copyright infringement.