Combined masking adds most when the two maskers are equally strong

Evidence

It must be expected that the increment in threshold, f1LT, will be largest when the difference between the masked thresholds produced by each masker is zero, i.e. when each masker produces the same masked threshold. If the threshold produced by one masker is much larger than that produced by the other, then the masking produced by the maskers together is likely to be dominated by the masker that produces larger masking. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 619