Kyurem and Necrozma have different formes but how come you are only allowed one Kyurem forme but you can have the two different Necrozma formes; Dawn Wings Necrozma and Dusk Wings Necrozma? Althought Kyurem has a total base stat of 700 while Necrozma has 680, it's 20 in difference.
If you play Pokemon White and White 2 or Black and Black 2, you can get both Zekrom and Reshiram as well as two Kyurems.
If you play Pokemon Sun and Ultra Moon or Moon and Ultra Sun, you can get both Solgaleo and Lunala as well as two Necrozmas.