Couldnt see this fix mentioned here for Retribution, but it suffers from the same stretched main screen bug as Chaos Rising when you first start it in triplehead. Fix is easy (and same as Chaos Rising) though:
In XP:
"My Documents/My Games/Dawn of War 2/Settings/configuration.lua"
In Vista/Windows 7:
"Documents/My Games/Dawn of War 2/Settings/configuration.lua"
Within this file are the specific settings to change:
"horizontalmonitorcount" - (Default: 1) - The number of horizontal monitors in your Eyefinity setup. This will make sure the game UI is rendered on the center monitor.
"uiscale" - (Integer, Default: 100) - The percentage to scale the game UI, tweak this value to your liking.
You can also toggle Vsync via
setting = "vsync",
value = 0 (for off)
value = 1 (for on)
I note the UI is properly centered by default (I didnt change UI Scaling) on 5040x1050.