I've never heard of anyone making a multiprocessor machine out of two different computers and motherboards. I'm not saying that it can't be done, but from my experience the way to do it is to get a dual processor motherboard and run two similar processors.

As far as sharing the internet connection, that is something very easy to do as long as you can network the machines. If you are using windows98 second edition or higher all you need to do is enable internet connection sharing and set the machine connected to the net as the gateway. Then you can surf from both as long as the gateway is connected to the internet.