Comparatively little work on combinations of therapies takes place in the research community. I suspect this to be a matter of regulatory incentives. For example there is little room for commercial entities to be able to make money by combining established treatments owned by other entities. Similarly for researchers, the world of possible approaches is balkanized by intellectual property, while the disposition of the majority of research funding is ultimately guided by the promise of a pot of gold at the end of the road. That pot of gold is much harder to obtain when someone else owns the therapies involved, and all that is being done is to apply them together. The edifice of intellectual property is a great evil, and this is one of many reasons why that is the case.
Given this long-standing state of affairs, there is at present little data to guide our expectations on the bounds of the possible when it comes to combining large numbers of therapies in search of additive and synergistic effects. Some people think that we should forge ahead in the matter of slowing aging: take every intervention with good evidence to date, and run large numbers of them in the same mice to see what happens. Should we believe that various ways of manipulating the operation of cellular metabolism demonstrated to achieve 5-10% life extension in mice can combine to double life span in that species? Intuition suggests not, but I don't think it to be completely out the question. Nor is it unreasonable to try it and see, given a rigorous approach to experimental design. Sadly, no established funding institution would go for this; it would have to be funded through philanthropy.
Why do I think that this is unlikely to produce large enough results to make it worthwhile? Because the evidence to date strongly suggests that the scores of methods of manipulating metabolism to modestly slow aging are operating on just a few core processes, such as autophagy. These are the stress responses that produce the lengthening of life observed in calorie restriction, and we know that these mechanisms don't produce anywhere near the same degree of life extension in humans as they do in short-lived species. Everything is connected to everything else in cellular biochemistry. A given interaction between two proteins can be influenced by adjusting levels of any number of other proteins, with widely varying degrees of effectiveness and side-effects. So most methods of slowing aging are different views into the same mechanism of action. The few combinations of approaches tried to date, involving only two methods, have resulted in mixed outcomes. Calorie restriction and mTOR inhibition may be additive, while growth hormone receptor knockout and mTOR inhibition interfere with one another, for example. That gives little insight as to the rest. It is hard to predict other results, beyond noting that a majority of interventions do appear to function through enhanced autophagy, and thus we might expect them not to combine in an additive way to any great degree.
What of the SENS rejuvenation biotechnology approach to aging, in which independent fundamental forms of cell and tissue damage are repaired? How will repair therapies combine? In this case we should expect additive effects: removing damage should be beneficial in proportion to the amount removed, at least when considered from a fundamental, reliability theory perspective. The mortality risk and longevity of a complex system of many redundant parts is dependent on its current load of damage. At this point we have no idea as to how that will turn out in practice, however. The contributions of different forms of damage may be significantly larger or smaller than one another. The results of two independent root cause forms of damage are not themselves independent: they interact, and probably significantly. Functional decline in one system spurs greater damage and functional decline in others, which is why age-related degeneration accelerates greatly in later life. It is a complex business. It isn't unreasonable to think that in some circumstances the results of rejuvenation therapies A and B will be indistinguishable from A alone, or that B will never achieve a great deal without being combined with C.
Can we envisage a world in which repairing cellular senescence alone produces no extension to life span because other, largely independent chains of damage and consequence are still life-limiting for old humans? That is becoming increasingly hard given the evidence to date for reversal of numerous age-related diseases to result from removal of senescent cells, not to mention the PAI-1 mutants who exhibit increased life span - but we know far more about senescent cell clearance than we do about any of the other SENS strategies. No-one is in a position to do more than make educated guesses about the results of combining senescent cell destruction with removal of mitochondrial DNA damage, or with clearance of specific lysosomal aggregates. Beyond "two should be better than one, but perhaps not in some specific cases" everything else will remain a mystery until the biotechnology is ready and the work is carried out. Making predictions seems a fool's game, given the degree to which the people closest to senescent cell research have been surprised by the scope and size of benefits observed in mice over the past few years.