Explanation for the Cap/Peggy story.
Thanos was right, but only kinda/sorta. Time isn't actually linear, but is only perceived that way because we experience it in a linear fashion due to the fact that we are all mortal, are born, age, and die. But time just...is. Existence simply...is.
So, yes, there can be two Caps in the timeline at any given time because they are not the same Cap.
Example: in The First Avenger (or was it The Winter Soldier?) where Peggy meets with Cap and tells him about how she lived a life, got married, etc., she's a little weepy. That's mostly because she remembers the pain of thinking he'd died, and is sad for the pain he'll have to go through over the next fifteen years. But during that time, her Cap is alive and well, waiting at home.
I believe that Peggy knows about the time travel and kept it secret the entire time. For that matter, her Steve Rogers may even have gone completely underground and gone by a different name altogether. For all we know, he's Steve Carter and that's that. But she can't tell Steve in the future after he thaws out, or it could create a paradox. Although, to be fair, you could just as easily argue that the concept of the paradox still depends on the linearity of time, rather than it simply...existing.
But yeah, they have their life, raise their kids, live happily, and Steve largely stays out of Peggy's way with respect to the development of S.H.I.E.L.D.