RCA bridged connection

[Substance]

Hi guys,

I currently have my 360 connected to my tv through HDMI but I am using the official Microsoft adapter to gain an RCA connection to connect my audio to my surround sound

I also have a ps3 which uses hdmi and also uses RCA the problem is that my surround sound only has 1 RCA input, I recently bought a cable which I thought would solve my problems,

Here is the cable :-

The cable goes from my 360 to the surround sound system and the PS3 RCA cable plugs into the back of it,

The problem is that I can only get audio from my 360 if the ps3 is turned on,

Does anyone have any idea why this is the case and can I do anything to stop it?

Thanks

*edit* After a little testing I have discovered that I can only get sound from either device if they are both on at the same time, not ideal really