This is the hardware setup I used for the first Extra Life event in Rift. Five accounts on two computers, with the main keyboard and mouse to the left controlling everything. I have three accounts running on the computer to the left, with one main character and the other two characters shown with picture-in-picture at the bottom of the screen. Similarly, I have two accounts running on the computer to the right. I can freely move the mouse cursor between the two computer screens with the single main mouse, if I need to control one or the other specifically. I can mouse click on any of the picture-in-picture characters to swap to a new “main” character on each computer.
In simple terms, when I press F1-F12 it is routed to all accounts on both computers. Mouse clicks are also routed, if I hold down Alt (useful for questing). Some keyboard buttons are a little special. ‘F’, for instance, makes my main character auto-run forward and have all other character auto-follow her. Ctrl+Shift+F just put them on auto-follow but does not start running. Another button (~) makes all characters assist the main character, and yet another button makes them issue the in-game /interact macro.