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.

I'm four years too late replying to this post, but even at the time this question was asked, it had actually been answered six years previous to when it was posted.

In short; the reply that was initially provided is completely wrong, and it's very much possible. Granted, it's more or less beneficial depending on what you want to do. There are a number of articles online you can find just by googling.

Like every automated task /job /project - it takes time and effort to set up. The steps necessary may seem daunting, but again, I think it depends on what you want to do and how important it may be to you. Once set up and completed, though, I think it could be worth it. Here is an article that specifically addresses your question:

From the article: "Apple surprised almost everyone with the introduction of Nodes and distributed processing with Logic v7."

http://www.soundonsound.com/techniques/networking-macs-using-nodes-logic