If they were small fish i would give it a go but be perpared to move one if need be. I have had agressive fish in together before but they were all brought up together and all grew at the same rate they do fight and chase each other for a day or two when they first go in, they just had to find there own little peice of the tank but they all still commune together in the middle. When one get the poops with another the just go to their little spots and have a time out. Also the dare not go into the others spots in just a trial and error. But if they are bigger fish (got to adolesent stage) i would not do it at all they will kill each other. Only try if you have a tank available to move one into. I have an oscar that was really small and he was brought up in a tank with neons (oscar was only a little bigger than the biggest neon) and he has never touched them they have been together for 12 month now and all still there.
When I asked my LFS, they said that they've seen it done and with enough driftwood and stuff for hiding spots, the fish will be fine. I'm not completely sure because arowanas are really expensive and i'd hate to have one die on me. Both are really pretty fish that I would like to have in the same tank....should i 
or should I not 
?
