Overwatch is not an MMO, and our MC here has max survivability anyway.
And you don't need an offtank if the main tank takes 0 damage no matter how many enemies she's up against (99% of game content doesn't require an offtank to begin with). You need dps to kill faster. Or maybe a healer, especially since they usually have defense buffs. Tank + Healer combos have the easiest time filling up groups, too.
Double tank will somehow turn out great for these two because it's based on a web novel and all the dumb decisions they make always turn out well. But if IRL your friend says she plays a tank, and you want to play with her more than once or twice a week, pick anything but a tank.