I don't want to be Debbie Downer, but I think you will have a chore enough keeping the achilles alive that added stress of another tang is not wise. That is, if you add them together (wisest for aggression issues, but not, IMO, for general health in this case). If you are set on an achilles--a beautiful fish I may attempt some day--then I would focus on him first and get him housed and happy for a year or so, then add another tang. The chevron should be benign enough aggression wise to be added later. But the achilles will own the tank, likely.