Not exactly the same scenario, but recently I was impressed at how two instances of MC could do different jobs with different libraries on the same laptop at the same time.
I was traveling to a funeral, where I was to show a continuous slideshow of random family photos, while also playing a shuffled playlist of vintage music (music and slides not related, just both happening at the same time). To reduce the gear I needed to tranport, I hoped to do this with just one laptop, audio to the PA system, video to a projector.
So, I tested at home and this worked: On a laptop I loaded a bunch of music files as one library, with a playlist set to shuffle. I loaded a bunch of photos as another library, also with a playlist set to shuffle. I set MC to allow multiple instances then closed it. I launched MC, loaded the photo library, started the slideshow, looked good. Then I launched a second MC, loaded the music library, started the shuffled playlist, sounded good. The two MCs behaved independently.
It all ran perfectly, except the slides eventually stopped because I hadn't set the photos MC instance to Play with Repeat. This was not a problem for the music MC instance because there were more songs than time to play them all.