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
IMHO put the energy into getting fixtures without the shared neutrals, or convincing the customer not to go with this particular design.