I have read the Cooperative Editing instructions (circa 2011) on the wiki, and could not find the "native_server" executable as I did not compile my own source.
To attempt to set up a multiplayer server, I follow these steps:
1) Start sandbox on both laptops using "sandbox_unix -t"
2) On one computer, click Multiplayer and then Mastermode
3) On the second computer click Multiplayer, then the server IP for the other computer
I then get the following error message:
How do I change the protocol for either of the computers so that they match?"you are using a different game protocol (you: 9, server: 7)"