Macos – How to the computer use 300% PC processing power

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:

1. 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.
2. 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+.

A unkillable process is usually caused by one of two things, either the process is being traced by debugger, or (much more commonly) the process has a pending I/O request to some device that is stuck. In the latter case the culprit is typically a disk device that's throwing hard read/write errors or has gone silent and the kernel hasn't given up on it yet. But it could be anything connected to the system bus that's having problems.

For USB and FireWire devices, disconnecting the peripheral from the system will often clear a stuck process. But if the stuck device is a video card or something else not hot-pluggable, a reboot is your only option to clear the process.