Same bodyshaped tangs normally cannot be kept together unless they are introduced together.
I have chevron tang, scopas tang, yellow tang, purple tang, sailfin tang, living happily together.
Monitor a few days first to see if the fights are getting lesser. Feed more to distract the brown tang from attacking te yellow tang.
Better still, catch the brown tang using bettabox and by putting bait. Don't feed them for a while.