Is it possible to combine processing power of 2 computers?
Not transparently where a running program can somehow use the second machine to execute code, since they are logically separate with no way for the cpu to communicate or access each other's memory.
That doesn't mean you can't combine processing power:
- Specific software might have components that can execute on other machines, e.g. protein folding, SETI @ home. These tend to be specialized, i.e. you can't start up Excel and tell it to use another computer for computation.
- If you are doing processor intensive tasks, you could use the secondary machine to run them, e.g. encoding/recoding a video stream.
If you are looking to harness the secondary computer in any way, being able to remote control it is crucial. Two ways to do this are via some sort of remote access (RDP, VNC) or alternatively something like synergy+.
If what you are looking for is a method of combining the processing power of two PCs into one, the "easiest" way to do it is to configure both of them as virtual machine hosts using software like VMWare ESXi (Be forewarned this will require the device to have compatible hardware) and creating a resource group or cluster and creating a virtual machine that uses the resources of both computers. This is NOT going to get you a full 2x speed (You'll lose resources due to virtualization) and is a limited solution due to likely compatibility requirements but it is the most "correct" answer to your question. The virtual machine will act like a single PC with the processing power of both hosts minus the overhead required to sustain virtualization.
One of my most used lines - Yes and No!
Yes it is possible - for certain applications that are designed to work this way. (Commonly known as a cluster - Further reading here)
No it is not possible (at least as far as I know) to take two off the shelf computers, "tie" them together and get the combined memory, processing power and everything else.