Birth Of A New Legend
Member
I believe that the reason why they can use kamui is probably either because they've got one of obito's collection of sharingan which I believe are all the same sharingan (I say this because in the kage summit arc after naruto and sasuke briefly fought it showed obito replacing his arm plus he had his mask off which means he most likely replaced one of his sharingan and another occasion when obito replaced his sharingan is after he fought konan and used izanagi right before the war he not only replaced one of his sharingan with a rinnegan but also replaced his blind sharingan with another one and was still able to use kamui with his sharingan hence why I say that obito somehow made copies of his sharingan so he can still use kamui etc) or another theory I have and it sounds far-fetched and possibly stupid but I believe that shin, the mini jubi looking thing and the unknown uchiha might have a genetically modified sharingan which might enable them to use the abilities of all/almost all sharingan but that's just my opinion no matter how stupid it sounds.