It's on page 189 of the Owner's Manual:
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Video Output
Select the HDMI monitor connector to be used.
Auto(Dual)
(Default):
The presence of a TV connected to the
HDMI MONITOR 1 or HDMI MONITOR 2
connector is detected automatically, and
that TV connection is used.
Monitor 1: A TV connected to the HDMI MONITOR
1 connector is always used.
Monitor 2: A TV connected to the HDMI MONITOR
2 connector is always used.
- If both the HDMI MONITOR1 and HDMI MONITOR2 connectors are
connected and “Resolution” is set to “Auto”, the signals are output with a
resolution that is compatible with both TVs. When “Resolution” is not set to
“Auto”, video may not be output. In this case, set a resolution that is
compatible with both TVs. (v p. 194)
You can check which resolutions are compatible with your TV in “HDMI
Monitor 1” and “HDMI Monitor 2”. (v p. 247)
- This can be set by pressing HDMI OUT on the main unit.
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My interpretation of this is that you either need to manually set the resolution to the highest common resolution of both units (the projector and monitor), or leave it on Auto. If set to Auto, it will try and detect the highest common resolution and use the same resolution for both units. If one of the units is off, then it should detect the highest resolution of the powered on unit. Auto-detection does not always work perfectly, so experiences may vary.