I'm going through the trouble of putting up a page just in the hope that someone reads this, follows this advice, and saves themselves a lot of time and needless screwing around.
This is simple, add xinerama to your global USE flag in /etc/make.conf if there is ever even a remote possibility that you might run 2 monitors in the future. In other words, always build with xinerama support. I just added a second monitor and have spent 3 days wasting time to get everything working right (as in true xinerama support in KDE). Save yourself the time when setting up a desktop machine and just add the use flag as it has no ill side effects if compiled in and will save oodles of time otherwise with recompiling and general messing around. Then, all you will have to do is modify your /etc/X11/xorg.conf and restart X.