As a first step, atomists might seek to fortify Mere Addition by appeal to Huemer's Modal Pareto Principle ('In Defence of Repugnance', p.903):
For any possible worlds x and y, if, from the standpoint of self-interest, x would rationally be preferred to y by every being who would exist in either x or y, then x is better than y with respect to utility.
Like 'Mere Addition', this principle is clearly atomistic: it considers each life in isolation ("from the standpoint of self-interest"), and leaves no room for holistic 'big picture' considerations. I don't mean to deny that it is an intuitively appealing principle. It serves well to highlight an intuitive feature of the atomistic view. But because it is so transparently atomistic, it won't have much dialectical force against those of us who find value holism antecedently plausible. We may simply ask: "why restrict our attention to self-interested standpoints?"
This claim may be given a more principled grounding by appeal to an independently appealing conception of ethics as fundamentally person-centered -- call this "metaethical individualism". (N.B. This is not merely to reiterate the first-order normative claim that promoting individual welfare is what's desirable. Rather, it is to further specify why - or on whose behalf - it is desirable.) The metaethical individualist objects: holists concern themselves with 'the world as a whole', when really the only entities worth ultimately caring about are particular individuals: Tom, Harry, Sally, etc. "Morality is made for man, not man for morality," they insist. Ethical acts are ultimately called for on behalf of particular persons. To aim at anything "larger" is to miss the point.
If this individualistic conception of ethics were correct, it would provide a strong, principled objection to value holism. But my previous post argued that it is not correct. Assuming the arguments in that post succeed, it follows that even atomistic total utilitarians should ultimately conceive of themselves as what I call 'world consequentialists'. They should reject metaethical individualism, even though the content of their axiology is "individualistic" in the sense that they claim that what's good for the world is just to promote individual welfare.
Rather than a deep, principled divide between person-centered atomists and world-centered holists, we find that both atomists and holists are ultimately concerned to improve the world. (So we might say that both are "metaethical holists", in this sense.) They simply have different first-order views about what this involves.