This is not an OpenBSD question but since you asked and I toyed with
this just yesterday...
To make dual head work with my ATI Radeon 9000 (on a T41) I need only
one line in the "Device" section:
Option "CRT2Position" "RightOf" #(or "LeftOf")
And as my screens are different sizes I also added
Option "MergedNonRectangular" "true"
I found that the device will run both screens without xinerama or
telling xorg there are two screens or any such thing - it all happens in
hardware - so I found I didn't need to confuse myself, or my machine,
with anything more than this. Two lines in the device section was all i
needed; I didn't add another "screen" section nor alter the
"ServerLayout" section. It just worked :) Your mileage may vary.
I found both "man 4 radeon" and "man xorg.conf" useful.
