IIRC I saw someone on another forum years ago had the same problem and wasn't happy with the results from a simple resistive mixing circuit.

Haven't tried it myself and result may differ depending on the sources, but it's easy enough to try (~5kOhm on center of each source with the
other ends tied together and to center of input. Repeat for each channel (L/R))

I think he went with a small VOX (voice activated switch) kit that had a dual pole dual throw relay and so could switch from a default to
a secondary stereo source as it sensed signal on the secondary source input.

I think it reacted and switched in less than 0.1 second and would hang on for up to a couple of seconds (both adjustable)

Also I think he had to slightly mod the kit as that wasn't exactly how it was intended to run/operate, but I think it was just a matter of rerouting the in/outs of the relay a bit.

If one source is - consistently, and not adjustable - much louder than the other the maybe stick a simple resistive divider on that input/aource?