30 gallons is way way to small. For right now your okay with the one arowana, but I'd try to get a 180 no later than January. As for having two, I think I've already sort of answered this question, but lets just say had a 240 gallon tank and you wanted to add that 4 incher. Yes, the very strong likelyhood is they would fight. Usually and probably in your case, the larger would be the aggressor. It can be real unpleasant to watch. I really wouldn't recommend it. And I say this as one who actually has two now and have kept two since the 80's. (not the same two). But this is not your real issue as I've mentioned. I really don't want to discourage you from your fish, but rather, I'd like to encourage you to find at least a 180 gallon tank for your ONE arowana. You can also put other fish in with him of course, just maybe not another arow. You might look for one used. I got a 180 gallon set up off of craigs list with EVERYTHING (filters heater hoods, stand, substraight, and even fish) for $300.00. Something to think about. Hope I wasn't to harsh.