Although this is one mode of operation, it would limit options, because 6/12 must always use the same comparator. 5/12 gives the benefit of using two different comparators (_OC4D and _OC4A) running on the same high speed timer.
It should be possible to use the core timer, but run two completely different generation patterns on the different comparators, since control of the comparators is what is used to manipulate high-frequency PWM sound generation. The effect at the speaker is to just mix the two signals.
If you instead just want louder, comparator 4D and 4A can be set up almost identically, but with opposing outputs. Although both are inverting outputs, changing the Output Compare Mode on one of them is the key to getting two matched but out of phase signals. There is a slight additional effort to store each “louder mode” comparator change in two places instead of one, but the calculation effort to determine the new comparator level does not need to be repeated.
@MLXXXp did testing on a number of these options; I think he was using some of the core functionality from some of the existing high-frequency PWM music libraries that are already available.