There is no guarantee that arowana and discus fish will get along well, but it's possible if you keep them in separate tanks. If the arowana is kept in a tank with other large fish, it may be harassed or even eaten. If the discus fish are kept in a small tank with the arowana, it may be able to get along.
If both Arowana and discus are of small size like 4 or 5 inches then it's okay to keep them together. Both discus and Arowana are fishes that prefer pristine water quality. Water parameters required are also almost the same for both. Slightly alkaline and slightly warm temperatures are the ideal water conditions for discus. An aro will easily adapt to it. But the problem is if an aro becomes the big size of 20 inches or so then they may find their food in the school of discus fishes. Moreover, discus is slow-mover with a gentle living style which becomes more stressed with fast-moving big Arowana. Better not to keep them together. Some may find it hilarious. But my vision as a discus keeper is that one should keep discus in a planted tank. They will like plants a lot. Arowanas are a big no-no for a planted tank. So I can't keep them together.
A snap from my planted discus aquarium