There is something else to consider.
Say troll X insults person A out of the blue. Person A replies in kind. And on it goes.
If Mike bans both of them, what do you think happens the next time troll Y insults someone out of the blue? Yes. He takes it for fear of being banned.
The site has then become a troll-fest because the abused and the abuser have been made morally equivalent. Both are bannable.
Bans should not be a tool only to quell unrest, but to correct injustice. Bans can show members that it is important to the management of the site that people are treated fairly.
Bans used only to silence unrest can be manipulated to ban whomever the troll targets. If membership sees that the mod(s) don't really care about who was right and who was wrong, they will lose faith in the process and we will end up with a troll-fest again.
Sometimes, carving the baby in two is not the best option.