Gamefreak Programed the game to have one Z-Move Per Battle. Since you are Battling Online, Two Ultra Necrozmas would be too overly powerful as the base stat total is higher than arceus himself. So add two of them and the opposing team is doomed, Making the other person rage quit most likely because you would be playing unfairly with 2 super powerful ultra necrozmas. However, You can have two fused necrozmas, one with Dusk mane Necrozma, And one with Dawn Wings Necrozma. The maximum is two different fusions besides ultra necrozma. You cannot have more than two fusions of one kind in your game or that fusion will be stuck that way because in order to fuse another sogaleo for example if you still have a sogaleo fused with necrozma, you must first defuse the necrozma attached to sogaleo to use a different sogaleo.