I think when Necrozuma exposed to the light generated by these two, it turns to resemble to the legendary mons from light refraction
>>32775872
Looks more like Solgaleo and Lunala are the hosts and that Necrozma is just a part of them or by all means when it is equipped to either Solgaleo or Lunala that it is the one in charge, though this is just judging by their appearance
>I think when Necrozuma exposed to the light generated by these two, it turns to resemble to the legendary mons from light refraction
Although, I think this would be a nice twist too
Or we can finally answer why the hell Solgaleo/Lunala was pretty useless while on standby on Ultra Dimension.