X11 : How to enable Xinerama?

Hi,

I have an iMac 20" with a second monitor attached, and I make heavy use of X11. The Tiger version of XQuartz has Xinerama enabled by default, meaning that X sees
my 1680x1050 and my 1600x1200 external monitor as a single 3280x1200 monitor. Leopard sees it as 2 separate monitors, and does not allow one to move X windows between the two. This makes X nearly unusable for me.

So.. can anybody explain how to enable Xinerama in Tiger?

Thanks,

Drew