, if you're going to pull 1 fish, you should pull all! Curing ich on 1 fish, while leaving other fish in the DT is kinda pointless, the fish will just be exposed to ich once back in the tank. And 2 tangs in a 20g QT for 8 weeks, plus other fish good luck!
BTW how big is your tank? Once Ich is in your DT, ya have to keep it fallow for 8 weeks to rid the tank of Ich while treating the fish in your QT. And unless you are going to QT EVERYTHING in the future, algae, inverts, corals fish etc, I'd just get your tank healthy
That means, feed a wide variety of food soaked in garlic and selcon, especially NORI for the tangs. I used herbtana and it seemed to help! Keep stock levels low, don't add stress! 2 big tangs, I hope ya have at least a 180g or so tank! Many people manage Ich just fine using these methods!
Obviously keeping it out of your tank is ideal, but there are options!