Is it possible to combine 2 computers into one and if so, how?

I want to share processing power, RAM, etc. between 2 computers; more specifically, motherboards with one mouse, keyboard, and monitor. Is this possible, and if it is, how can I set it up?

Is it possible to combine 2 computers into one and if so, how?house call

I'm not quite sure exactly what you mean.

If you just want to switch between two computers, controlling them with a single keyboard, mouse, and monitor then you want a "KVM switch."

If you mean that you want to offload processing on one machine to another, that's not generally done except in specialized fields like 3D rendering ("render farms"), web search (Google and Yahoo do this) and cluster supercomputing (which combines many commodity machines in a cluster to get supercomputer-level performance on the cheap). The latter is what it sounds like you want--but you don't really want that, as you wouldn't be asking this question here if you were in a field that requires that sort of thing. The closest a consumer is going to get is to make one of the machines a server that takes over any auxiliary responsibilities (DHCP, network shares, whatever) of the main computer, which probably isn't worth the trouble of setting up.

My recommendation is to take the best components from each computer, combining them into the best possible machine. RAM, especially, is a quick swap (if compatible) that really enhances performance.