if there is two VGA input and one Video input, which decoder suitable for this?
that means at least a 2:1 H/V input same time.
Something along the lines of the ADV7842 should be fine. Check out:
UG-214 Figure 13 where second VGA port is connected to Ain4,5,6. The syncs will attach to HS_IN2 and VS_IN2
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