Tangs of different body shapes usually get along OK, its when they have the same body shape they do battle for me. I would'nt put two in a tank any smaller than 90 of so gallons though, they need room to swim and grow. Alwasy stock a tank based on the fishes adult size not how big they are today, your goal is to keep them their entire life so make sure you have the room. I have had several tangs including a hippo and sailfin live past 13 years and many past 7 years.