Just spent some time trying to figure this out myself, and thought I'd post an answer in case anyone else finds themselves scratching their head. In my below explanation f1 cutoff refers to the actual filter cutoff, and CUTOFF refers to the big filter shared knob CUTOFF 1/2.
The reason the filter interaction is confusing is that it's seemingly non-symmetrical. What am i mean is this:
If you turn the CUTOFF all the way to the right (max), it will completely override the filter 1 cutoff. Notice how you can turn the f1 cutoff knob and there is no effect on the sound when CUTOFF is at max.
However, if you turn the CUTOFF knob to min, then the f1 filter knob is unaffected by the CUTOFF knob. Now you can turn the f1 cutoff knob and have complete control over the sound.
This all seems rather non-intuitive since when turned to max, the CUTOFF knob completely over-rides the f1 cutoff. And when turned to min, the CUTOFF knob has no affect on the f1 cutoff.
Even weirder, when CUTOFF is at the default position of half-way, the f1 cutoff knob seems to override CUTOFF when turned less then half-way (left half of knob), and is overridden by CUTOFF (ie has no effect on the sound) when turned over halfway (right half of knob). And when both are turned half-way, you get a very different sound then if CUTOFF is at min and f1 cutoff at halfway. So they are summing together when both of them are at half-way.
And actually this is the answer to the mystery. The filters actively sum together. So when both are half-way, you get a fully open filter. If one is min and the other half-way, you get a filter at half-way. Because the scale of the filters is logarithmic, think in terms of the knob percentage instead of hz when summing them in your head.
In my opinion it would be much more intuitive if the f1 knob had full control when CUTOFF was at max, and no control when CUTOFF was at min. Then the blue highlight of the CUTOFF knob would define the active range of the f1 cutoff knob.
