FWIW, when I contemplated doing this (using wireless audio) I planned on using an automotive speaker level to line level device that solves multiple problems- one huge one being that common or ground isolation issue. Again, the idea was use an off the shelf and relatively cheap design that contains voltage dividing resistors, and then a specially designed audio level transformer and also adds a potentiometer to adjust that line level output and sensitivity to input.
While the device is stereo, I found you could cut the circuit board right down the middle to get 2 mono units- to put into 2 different trains.
But here is the catch22 I ran into- Latency. The problem what that the electronics (I was using off the shelf Bluetooth transmitter and receiver modules) transmitting and receiving add a slight delay (in some cases- more than a "slight" delay) making it unsuitable. Again, if the intent was to copy and add to the original sound of the speaker in the lead engine- the problem was your second output was delayed and sounded minimally like an echo and worse, was even more delay.
@gunrunnerjohn is doing a great job of pointing out to you the power supply and other requirements to do that isolation.
Again, my concern is that I've yet to find modules with no delay. I've seen claims of no delay, but none thus far actually could play in concert with the source and not have a delay.
