you have to network them together first one being a slave thats not used and the other being a master that you operate off, then you have to use special programs, I would advise reading up on the subject, it isn't an easy task, check out 'Beowulf clusters', and it only really works well with Linux, it's even more difficult with windows, oh and you will need to know how to script within a os.
May I ask why you are doing this, because it is only advantagous if you do a lot of rendering or stuff that would work better with two minds focusing on different tasks, it won't work at all with such things as gaming really. Btw it also doesn't work well with two or more pc's, better to have 4+ really...