From what I've read on this site, from people who keep sharks, you want something circular, or oval, that's 5-6 times the length of the shark in diameter. That's a round pool that's anywhere between 25 to 30 feet in diameter, and a height of about 6 feet. And that's minimum. This is because the blacktip reef shark is an obligate ram ventilator and therefore must swim continuously in order to breathe. Whitetip reef sharks aren't ORV sharks, so they might be okay in something smaller, but since you want to put them together, you need the bigger pool.