Interesting idea but sadly not possible. Each mainboard is designed to be a single control unit, to attempt to link two would just give you an endless stream of Blue Screens as one board and then other tries to take control of the system processes. It could be possible if there was specific software created to do this but you would still need to program a single controller.

As to the PS2 idea, not going to work either for exactly the same reason, you cant make a single logic board share resources it wants to exclusively control.

You can buy mainboards that have more than one CPU slot, normally used for servers they are rather expensive and understandably large, but the biggest I have seen takes four CPUs and has 16 memory slots, I seem to remember the cost was several thousand pounds just for the board. It would only run a specific version of Windows Server as well, most versions of Windows are designed to run using just the one CPU (now of course with multiple cores).