Same as krichardson, keep feeding what the IT eats and soon enough the AT will follow. I, and I'm sure many others in this subforum have had datnoids that have gone on hunger strikes in excess of a month only to break down and take the foods that are offered in the aquarium.
Keep in mind these fish are all wild caught and even though they're young they're built to hunt other fish, it's in their DNA unlike some other fish that are several generations removed from the wild and will more readily accept frozen, freeze dried, pellet or prepared raw foods.
In my experience with datnoids going on about 6 years now, as long as one is eating the others will follow eventually. No healthy fish will starve itself to death on purpose and the survival instinct to outcompete the other fish should eventually drive your AT to eat what is available.