When you choose the "Parallel" configuration, the oscillator's section output is fed equally to both filters. The filter balance knob then acts as a mixer knob, as explained on the manual. In the middle position (12'o clock) you can ear both filters mixed equally. Fully turned left, you only listen to Filter 1, as you start moving it right, you start mixing in the output of the second Filter, and vice-versa.
As you can see on the routing diagram you posted, the "filter saturation" stage is applied to the first filter only.
With the "Split" configuration, the output of oscillator one is fed into filter 1, then the output of oscillator two (and the third as well, if turned on) is fed into the second filter. Each of these signals, on this mode is fed to its own VCA and you can then adjust the amount of "spread" using the "UNISON Pan Spread" parameter, in the EDIT menu.
All there is to it
