Off hand I cannot think of a specific prohibition that says 'you cannot connect neutrals from two different branch circuits feed from two different panels'. But I am sure doing so will create code violations and cause problems with system functionality.
At a minimum, if you have an AFCI requirement, such a tied neutral would present as imbalanced return current and cause the AFCI to trip.
How do you properly size the neutral conductors? Remember that either could potentially carry current from _both_ circuits.
Is this tied neutral effectively a parallel conductor with the feeder neutral? What happens if there is a fault on the neutral of the PV system?
Don't even bother trying to figure out in what circumstances such a tied neutral would be permissible, except for the fun of finding holes in the code
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IMHO put the energy into getting fixtures without the shared neutrals, or convincing the customer not to go with this particular design.
-Jon