My advice is have one or the other, but not both together.
Having both in the same tank is a headache.
I have a royal knife which I had to move to my tank at work.
Clown/royal knifes require to be feed regularly if you want a peaceful community tank with arowana. Trips away for a more than 2 days will result in the clown knife becoming aggressive and greedy for food. When there is no food, your arowana or other fishes tails and fins are the perfect target for your knife fish to take it's frustration out on. A territoral arowana vs a hungry clown knife = battle royal and most times arowana comes off second best. Clown/ royal knives have an incredible jaw that can do some serious damage.
Clown / royal knife on it's own is fine, and can be left for around a month without eating. Same for the arowana on it's own.
My advice is have one or the other but not both together.