I can't see how you could possibly hook up another computer to show a view of both rear view mirrors. To be honest I fail to see any point in having the number 2 computer at all.
What comes to the rest or the question, if I understood it correctly, you simply need to set up one of the computers as the server and connect to it with the other. Or you could use the third computer to use as a dedicated server, whatever rocks your boat. But basically with computer #1 start with "multiplayer ->start new host". Then when the host is up get your command prompt (press windows key and r, type cmd, press enter), type ipconfig and press enter. You'll see the ip address of the (game) host computer. Then simply connect to that host with the other computer by going to "multiplayer -> join specific host -> local network" and type in the IP address to the IP address field.
And for the love of god learn some punctuation.