For those, like me, that used to use the function on Sound Assistant that let you play two music sources at the same time, you still can!
Kind of by accident I found that, even though the individual app volume options do not appear on the pop up volume menu, you can still open Sound Assistant separately and adjust the volume there and have two sources playing. I had to confirm my separate music app choice again (YouTube Music on my case) and then it worked.
Hope that helps anyone who may be missing that feature!
