I depends on how you want to tune your quiver, and that in turn depends on for example how wide the spectrum of conditions you sail in is. K79 and K89 will be rather close, but distinctively different still, especially for a heavier sailor. Since the K89 is already in the quiver, the question is how small you want to go with the next size down. PA74 will for _most_ (not all) people in the 85-90kg range be for quite a lot of wind and maybe even more importantly, for very steady winds. K79 will for someone in that weight range still handle a lot of wind, particularly on flat water. In waves it will offer a more maneuverable alternative to the 89 when waves are OK, but when the wind is maybe not super strong and solid.
So, what K79+K89 quiver does is to focus less on the extreme ends of the wind ranges but more on giving tuning options in the important mid range.
A K89+PA74 will cover extreme ends of the wind range well, but leave somewhat of a whole in the mid range (or at least an area of "thin coverage").
K89+PA80 will be kind of a compromise.
But quiver planning is an individual thing. I personally think many people "spread out"
their quivers to much. I'd rather have to all round boards with high respectively low wind bias than two "extreme" boards. But thats me.