OK, I just figured it out ... I think. As far as I can tell, you have to have either 2 web subscriptions to do this, or a CD version.
It helps greatly if you can access the Remote computer via Remote Desktop Connection or VMWare or some other remote computer viewing program.
"Local Computer" is the computer on which Render Server is running.
"Remote Computer" is the other computer you want to use as a render slave.
Install AM v16 on both computers.
On the *Remote* computer, find the RenderMessenger program in the v16 install directory and make a shortcut to it on the Desktop.
Run RenderMessenger on the remote computer.
It will ask where RenderSlave is. Point it to the RenderSlave program installed on the *Remote* computer.
Then it will ask the IP of RenderServer. Type the IP number of the *Local* computer where RenderServer is running.
Make sure you configure your firewall to accept connections from RenderMessenger on the Remote computer to connect to the Local computer.
Once RenderMessenger is running on the Remote computer, switch back to the Local computer.
On the Local computer, fire up RenderServer and it should list the Remote computer in the Slave List window.
You get a new render node that can access as many as two cores on the new node for only $80 extra dollars. That's only $40 per extra processor, unless your new node only has a single core