It is possible to connect two similar input devices that will work simultaneously.
I did it myself when I played FIFA with my friends, and connected two mouses to my computer.
The trick is that you cannot work on two different windows at the same time, that is - both keyboards will input to the window which is in focus.
However, there are gadgets out there that will enable two users working simultaneously on one computer, just like a Terminal Server allows two (or more) users to work on different desktops though there is only one computer (for that matter, a single motherboard) behind them.
Unfortunately these gadgets are costly, and I guess this is not what you are looking for...