Help

I’m not sure if this is a fanfiction or not but I remember two boys (I think in highschool) travel to a different universe where they meet themselves as adults. The adults are in a zombie apocalypse and the two boys couldn’t leave for a short while. Soon a portal opened And I remember one of the tried to pull the older one in (by the wrist) but the older one pulled back, saying smth abt this was their universe or smth.

Idk I clearly remember it and I felt hella bad with their (the adults) situation