How to prevent from changing volume when using 2 to 1 RCA connector I prepared scheme: 


(Amplifier I use is Pioneer A-307R  http://www.pioneer.eu/eur/products/archive/A-307R/page.html

TV Samsung UE40D6750 http://www.samsung.com/uk/consumer/tv-audio-video/television/led-tv/UE40D6750WKXXU)

I connected to Apple TV both HDMI and TOSLINK to be able stream only Music (without need to have TV turn on).

The key point here is the "2RCA to 1RCA" connector, which makes the volume up when one of the device (TV or AppleTV) is turn off. (Or makes the volume down when one of the device turn on alongside other).

For Example: 

While you watching TV on TV alone, when you turn on the Apple TV the volume goes down, when you turn off the Apple TV it again goes up to the original level.

Could you explain me why it happens ? And how can I make it work without the volume going up and down?

I thought of ARC system where I will get rid of the TOSLINK cable, and plug the the HDMI directly to the Amplifier, although my Amplifier doesn't support ARC , and I doubt that there are connectors that make it possible to be compatible with ARC and have RCA output.

Thank you very much for your time and help.
It is dropping in level because the end of an audio connection usually has a termination point. and/or the signal is being routed to electronics that are adding slight resistances to the signal.  (in simple terms there is a cap on the end :-)

The least solution, would be to use an isolation transformer to one of the connection point, then one of the Yed signals could be isolated from the other device.

The most solution would be to use an audio distribution amp. Or to  route through a device that would passthrough the audio (unterminated) Like some amps or pre-amps. or to use other devices (like a equaliser) that would have 2 seperated outputs.