You need a software multiplexer (a multiplexer is a device which sends keystrokes to multiple devices at once). Keyclone, octopus, auto hotkey.....etc. can all do it.

To run on 2 different computers you need a program that will communicate over your network to both systems. I would recommend keyclone due to its ease to setup.

Once you have a program running that sends keystrokes to both computers and to both wow instances you will still need macros to help control everything properly. For instance you dont want both toons to just blindling walk forward at the same time. Unless they are completely synced and facing the same direction you will have them start to separate. So having a follow macro is absolutely necessary.

Get keyclone up and running on both systems and make sure they both respond when you hit a key. Once you are to that point then check out the macro section for the basics to get your duo running and gunning together.

Good luck!