-For Obito I would say its b/c he has Senju DNA which maybe a requirement to keeping Sharingan from losing its light.
-As for Kakashi, I don't think he can personally. While an Uchiha may lose their eyes from using sharingan, Kakashi on the other hand would just die. He doesn't have the blood of an Uchiha so he shouldn't suffer from the same consequence.
Obito has Senju DNA. I can only conclude that Mangekyō Sharingan are eternally linked, regardless of whether or not they're possessed by the same person. If one awakens, so does the other. If one goes blind, the other will meet the same fate. If Obito's Senju DNA prevents him from going blind, Kakashi will also be blessed.
kamui puts less strain on the eye than amaterasu (kakashi and obito never bleed after using it, only once has kakashi bled)
plus susanoo probably accelerates the rate of blindness
plus as others said obito has senju dna and kakashi has rarely used kamui uptill the war