Kekkei genkai and their related techniques cannot be taught to or copied by others. However, kekkei genkai can be given to others. Dōjutsu, or other kekkei genkai isolated to a single organ, can be transplanted into the body of another, as was done with transferring Obito's Sharingan to Kakashi. SO Kekkei genkai Is someting special Itachis and Sasukes body was uncapable to use it without a cost and obito and Kakashi has very similarly Kekkei genkais and shered just 1 of dojutsus(1Sharingun) So Kakashi and Obito just dont use full power of sharingun thats why they canot get blind If one of them get both sharinguns It will unlock new Powers and will pousibly get damage by uzing that power.