It is definitely the table audio tracks that play for each table within PinballX. It's distinct music that is only in the table selector of PinballX.
It's certainly not running another FP instance (for one....this computer is from 2007 and would meltdown running two FP tables at once haha)
I'm not sure why it's creating backglass window twice. It does a kind of a clunky switch when I select a table. It starts to load, goes black, but pinballx re appears within about 2 seconds and stays on screen, then it finally goes back to the FP loading screens and launches. I bet it has something to do with that but as far as I can tell I have all the window settings correct.