So to be honest not sure if it'll work, and I don't have much of a way to testing it since I've never even been into an arena before, and my guild seems to busy to join a party for a bit, but my gut feeling says that it will work.
/target [target=arena1, harm] arena2
/target [target=arena2, harm] arena1
It also isn't full proof since it won't work if you have something else targeted other than arena1 or arena2, and I know there is a way to patch that flaw up a bit, but apparently my brain is too fried at the moment to come up with it. Maybe Quinafoi will appear to fix my macros as usual. If not I'll revisit it tomorrow sometime when I actually have had some sleep.
Edit: I think I figured it out. Should test it. Bed time.
/target [target=arena2] arena1; arena2