Depending on how long the dietary complication will continue, you may consider sequestering each in separate rooms with their food dishes, then shut the door until they get bored and finish their food.

Fortunately, we didn't have that problem with LV and Connor during his decline -- all his meds were dispensed directly to his mouth, and we otherwise fed them the same (expensive high-quality) diet-mandated food.

Good luck resolving this snag!