it's impossible to say why it would have two video and only one audio input. maybe, one is an output to additional screens. when you add multiple screens, the signal can be degraded when it is split too many times, or it travels too long of a distance. this usually results in horizontal lines or a blank screen. they sell video amplifiers to fix this problem, but many screens have one built in.

as for the ir headphones, the best way is trial and error. many are interchangeable, but some are not. expect to pay a little more for 2 channel headphones (they are usually for dual source headrest kits)

James Stanton