My understanding, contrary to popular dictum, it's about the stronger sub, not the weaker sub.
When the weaker sub drops out (the weak link) one is left with the stronger sub. The question becomes, in the case of the lower octave where the weaker sub drops out; what happens? In my opinion, two thing will happen, a null can develop because not as many subs are filling in the voids room acoustics create and second, the question morphs, will the stronger sub be able to stand alone during the moment it's called upon to carry the sound track by itself? At the time the weaker sub drops out, one loses the synergistic addition of the weaker subwoofer and then, the sonic chain becomes about the stronger subwoofer as the graph collapses accordingly.
Yes, the weaker sub will fill in the mid-bass range but as to which HPF setting to use, will depend on the phase alignment of the ported vs the sealed, and what distance each is placed from the MLP: not to mention the affect of cabin gain on the left side of the graph. Stating the obvious, the user wants everything to arrive at the same time.
I see mixing sealed with ported being a case of dealing with phase alignment issues as I would suspect one would want the HPFs to be set equally.
I'd run a series of sweeps, using a continuous phase correction (make fifteen degree changes, measure and repeat) to see how changing the phase affects the graph. REW will pickup and display a change of a few degrees. It's surprisingly sensitive in this way.
What's the purpose of the exercise? What is it you don't like about the graph? And forgive my above if you're already onto what I was going on about.