both fish can grow to be very large. while the black ghost is semi-aggressive I have seen an Arowana take food out of another fishes mouth. Also seen it smack the glass tops of its tank.This fish was found on the floor dead one morning after smacking the weighted glass tops off its tank, they are incredibly strong. Even though they inhabit different levels in the tank I don't think they would be goodtank mates.
The silver arowana can get to be 3 foot and a 200G tank is recommended for it.
The black ghost Knife can get to 1-1/2 foot with a recommended tank size of 50 gallon for it alone.
They are both very interesting fish but I'm afraid all you would get to observe from them would be fighting.
ide say they will be fine ate the same size,
but the arwowanna** will out grow the black gost knife fish,
the one at sd seaworld is 4', but they have it with a 1'6" peacock bass,
so maybe with a big enough tank