Sounds like a fun project! 

Tearing is defiantly an issue with some projects that I have worked on, I don't think that the number of screens has anything to do with it, I've worked with 4 and even 8 a few times with good results. However, I have always seen a bit of tearing, and it seems to be worst on mac os. This may be due to driver support from the graphics cards on different platforms. The 5770 was what was in my old mac, and it did decently, but oddly enough my older laptop still beat it out in framerate tests.

If you have a windows machine available, I would try that out to see how it preforms. Also have you tried any of the 2.0 betas? I have noticed some great improvements for certain things in the newer versions. What renderer are you using in setup? P2D? P3D? Have you tried OpenGL as a renderer? what about drawing with OpenGL? ( http://wiki.processing.org/w/Advanced_OpenGL)  Are you using an animation library? 

Here's a few links that looked promising that a quick google search came up with (I searched for 'java' rather than 'processing', usually gets more results, and the answers still apply)

http://stackoverflow.com/questions/2114455/skipping-and-tearing-in-java-animation

http://www.java.net/node/655913

http://www.gamedev.net/topic/629869-java-swingawt-screen-tearing-and-double-buffering/

Let's see some code! :)

good luck!

ak