My jar and 15 clown loaches have been together in a 240g for the last 9 months. Since the time they have been together the jar has grown from 5 to about 16 or 17" (eye balling) and loaches average size is 3-4". No aggression from jar except for occasionaly minor irritation when CL steal his food which are mostly hikari food sticks. CL does a good job of eating and picking on the jar's left over foods. Jars are really messy eaters. I was told by a friend to refrain feeding live feeder fish to jar not only because of the obvious possible introduction of disease but it may make the jar aggressive, brings out it's true hunting nature or it may get addicted to live feeders and stop taking dry or frozen foods.
Some people would say its worth the risk, and you could try it if you have the means to separate them at the first signs of any aggression, but I would still say no. In general jars are very aggressive towards other fish when kept in tanks, even more so towards their own kind. The chances of finding even one jardini that will live peacefully with another arowana are pretty slim, as for two...