So after some thought I think I got something:
70 hz LP + 100 hz LP = 42 hz LP (40 hz to 45 hz) if you add the half of 70hz LP to the half of the difference between the two (which is 15hz)
So if we take the lower low pass crossover and add up the half of the difference with the higher low pass one ends up being something like this:
70 hz LP + 100 hz LP = 35 hz LP + 7 hz LP (1/2 of 50 hz - 35 hz) = 42.5hz
So in this case when you add both filtered signals they only add up half of the frequencies of between them. They stay at the middle of their halves.
So 50 hz LP - 35 hz LP = 15 hz x 1/2 = 7.5 hz
Another theory is that they could be like this:
For 70hz LP is 35hz(1/2) = 17.5hz
For 100hz LP is 50hz(1/2)= 25hz
To illustrate it better you just have to image the middle point between the halves of the two low pass filters crossovers you are pairing (since you are pairing them and they are Low Pass, they subtract each other).
Maybe I´m wrong but in some weird way I cannot explain it makes sense.