Life parallels?

I’ve been trying to grasp my ahead around the fact that two people could really be living two different lives at the same time like someone could be on vacation with their whole family happily while at the same time same date everything someone could be battling a terminal illness or be at a the scene of a car crash for for their now deceased family member. I wish i could read a book or an article on that too really try and grasp my head around it because the idea of that is just so crazy and devastating to me. (this is my first time posting something on reddit not sure if im doing it right 😭)