To answer the second question first, add the line:
Option "xinerama" "on"
To the "ServerLayout" section, right below the keyboard info. RH does some funky Xinit goofy stuff, but all of it can be overridden by creating a file called: /home/whoever/.xinitrc
and you should have example copies for all the common desktops somewhere that you can simply copy over and rename or, symlink to.
The freezing? dunno, xinerama does some weird weird things sometimes.