I recently got a StarTech USB3.0 Dock with DVI, HDMI and VGA outputs and the above method still works when using modesetting for the primary GPU and having PageFlip off on all driver definitions. Using evdi 1.3.43 and DLM 1.3.52.
So I have both external screens connected to the dock and both rotate ok. This doesn't seem to honour VSync, so glxgears reports about 290fps.
I have named the Driver Identifiers Card0, Card1 and Card2 and that seems to convince X to pick up the PageFlip off Option correctly every time, so no need to restart display-manager.service.
Stefan
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