Reply to 2 Midi Fighter w/ External Mixer and Soundcard

How would I set up 2 midi fighters with an external mixer and soundcard? I know that the midi fighters plug into the usb ports along with the soundcard. But are the main outs that go into my PA system coming from my soundcard or the mixer? Do the mixer the soundcard have to connect to each other via RCA? If someone could answer this question from a begginer that would be great!

Thanks!