Currently Doomsday only supports the extended desktop mode, in which case the DK2 should be appearing as a second monitor. In this case, Doomsday should detect the DK2 and move the Doomsday window as fullscreen onto that monitor automatically, making it unnecessary to change the resolution.
YMMV, though. As a fallback you could try the console command:
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YMMV, though. As a fallback you could try the console command: