that tank is WAY to small for a hippo tang, sure it is a small fish now, but it HAS TO HAVE more room than that. tangs cover hundreds of miles every day just hunting for food in the wild. this is going to cause stress which in turn is going to cause you all kinds of problems.
no idea on connecting them, but you could always just add a
sump and drain them both into it and run 2 smaller return lines.
