oddly enough, after obito gives kakashi his sharingan, he never deactivates the spare sharingan (to our knowledge) or his kamui eye either, even when he was seriously injured and recovering, even after losing 10 tails and super fatigued, hell even after kakashi pierced his heart, he didnt need his sharingan active to become jinchuriki.
Could obito be unable to deactivate his sharingan because he doesnt have both of his original eyes?
We see another instance of this when shisui gets his eye stolen, and after itachi casts izanami and loses his left eye, theres no need for him to keep his other sharingan active but he does till the jutsu is released.
Side note, this could further confirm that to obtain ems you dont swap your old eyes out, and that it is a fusion.