the x200 i’m using has enough omph to power OpenGL on both the builtin display and the 1600x1200 external display. unfortunately whenever i moved that glxgears window on to the second screen it would just show…nothing. googling around for that showed that a number of people had the same problem but nobody had found a solution.

repeated staring at the intel(4) man pages resulted in the hunch to try and switch from EXA acceleration to XAA acceleration

Section "Device"
Identifier      "intel"
Driver          "intel"

Option          "monitor-VGA" "VGA"
Option          "monitor-LVDS" "LVDS"

Option          "AccelMethod" "XAA"
EndSection

an X server restart later, and, voilá!, glxgears shows its gears on screen 2. problem solved.