If we are talking about negative mass, it may be that mass and inertia become more clearly different things. Something with "negative" mass might be "pushed" b

If we are talking about negative mass, it may be that mass and inertia become more clearly different things. Something with "negative" mass might be "pushed" by gravity but still retain normal inertia when touching other types of matter. So the two balls would still bounce off each other as normal, but the gravity interaction between the two would become weird and result in a net gravitational force moving both in one direction.