In general experience, discus can be shy when first brought home. You'd definitely want areas where the discus can lay low, and the arowana has it's own territory. Though it'll be interesting to see how you manage to arrange feeding both fish
However as I said before, it's a weary proposal. The water conditions are different for both fish, and the requirement levels on different scales.
It all depends on the temperament of your arowana. I kept a 3ft silver arowana with a pair of discus for 3 years and never had a problem. But i didn't feed it any live food mostly blood worms and pelets. At one time i had a up to 3 arowanas with my discus . i had a mean green Asian, a black and silver arowana. I had them in a 8f 240 gallon tank for all the first 2 years the i moved them in to a 470 gallon tank..