[Elecraft] K3(s) Headphone Splitter

[Rich]

I am planning to build a headphone splitter/switch so I can listen to 
one K3 in the left ear and the other radio in the right ear or turn off 
the audio from either radio.

Two questions:

Is there a commercial item for this purpose?

I know I will need an audio isolation transformer between each radio and 
the splitter/switch.   Any suggestions on what a good impedance value 
would be for those transformers?

Thanks

Rich

K3RWN