Q: Can I connect two MacPros  to increase processing power?

Not really.  They could talk to each other to send data but each one is totally independent of the other.

FWIW here's a similar thread in these forums (except related to imacs):

Combine processing power of two iMacs

This article talks about "embarrassingly parallel" kinds of jobs and Mac OS X built-in XGrid.

Science - Hardware for Science - Grid Computing

XGrid agent is built into Mac OS X, but requires Mac OS X Server software as Controller.

It's not possible to native we connect the Macs.  However, some video editing software like Final Cut Pro have a feature called cluster rendering where you can install a simple child script on the other machines and it will utilize their processing power through the network to render your video. I think there might some applications that do it for bigger web design projects as well.