I've been trying to remember which comic I saw this in. It was Superman and The Flash running through time till they got to this net-like thing at the end of time. I remember reading this in the 80s, but it was a beat-up comic, so it could have been earlier. Any help is appreciated!
The title of these comics are Superman and The Flash, with the first one having a bubble saying
The Man of Steel and the Scarlet Speedster embark on a deadly to CHASE to the END of TIME
The Man of Steel and the Scarlet Speedster run a deadly RACE to the END of TIME
It seems that there are two warring peoples from a similar species, a scientist from one of the peoples is attempting to merge the species but this would have a drastic effect on the humans.