Keep in mind that tangs are truly 'ick magnets'. You would have a hard tim treating them in a reef. Consider how much live rock...it takes up space so therefor there is less water. Some will say this doesn't matter, but think it over. Less water...less swimming area. It just makes sense, you add live rock, it takes down your 'gallons of water'.
Tangs get very big, I think 2 is one too many. JMO