try moving the display cable from the tearing monitor to the middle then set it as your preferred display.

Stick with it. It's totally worth it.

Seperate systems:
one 5870 w/ six Mini Display ports
&
6870s in Crossfire. (Two DVI + One Mini Display Port)
had the same issue with both now and then

AMD:
http://support.amd.com/us/kbarticles/Pages/Screentearingwithmultipledisplays.aspx
+
[H]ard|Forum:
"I heard of a some guys limiting their FPS to 60 using 60Hz monitors and that works for them but never tried it myself."
"3 display port connections is the only way to solve it"
http://hardforum.com/showthread.php?t=1750573

In my situation it resolved itself. Don't remember exactly how.
(6870s in crossfire)

here's 12.11 with NET4 if you want to try an older version
https://www.dropbox.com/s/mzqddpbpgipn1xr/AMD_Catalyst_12.11_Beta11_NET4.exe

Set preferred display to that monitor and see if it helps
http://img14.imageshack.us/img14/5586/ow14.jpg
In the past I have had to set the preferred display to the monitor I view the TV tuner on because of tearing.