Hello obsi,
The reason we advise against leaving both high and low level connected at the same time is that when one input is operating, either high or low level, and the other is still connected, the circuit will "bleed off" a voltage and send it up the line or low level input back into your AV receiver or amplifier, with unpredictable and likely negative consequences.
Even if you connected a switch to disable one of the inputs while the other was being used, you or someone else might forget, and cause damage.
Regards,
Alan
