I have a feeling that you really don't want the same values for both multi-part instruments. One instrument might be lower in volume or the filter might be set differently. On the keylab essential controller, there is a selection to control Part 1 or Part 2 or live. On a different controller, you might not have those functions. You might want to capture midi codes using a Keylab Essential controller (or other that has multi part selections) and see which codes are sent in order to target part 1 or 2 at which point, you might want to program two buttons to send those same midi codes to emulate the Keylab essential. You could also ask Arturia via a problem request for those codes and then map them.
Obviously I don't want the same values for both multi-part instruments in a Multi, and I don't want to use Keylab keyboards - not everyone does.
The Part-1, Part-2 and Live switches you mention are KeyLab-only features so are not available on a 'generic' controller like the Nektar P1.
What I am suggesting is that in 'Generic Midi Controller' mode the generic controller should control whichever part is currently
on top/active in AL4 - Part-1, Part-2 or Live. That does not seem too difficult to achieve as each of the three sections contains the same 10 knobs and 10 faders (albeit with potentially different assignments and different values). So for example, Knob-1 on the controller should always control the
visible Knob-1 on the active AL4 tab, not an
invisible knob on an
invisible tab.