Toes stepping is generally not wanted. From that stand point, the question would be . . . are we telling a story from many different sides and roleplaying the situations and events as they take place? Or, are we roleplaying and letting the story fall where it may?
With the former, they will be some sharing of information among the GM's, the players will have to separate that information as one big spoiler and deal with it as they would any other spoiler information - separate character knowledge from the knowledge the player has. The story will be more cohesive, timing will be an issue. One GM should have final say in the collaboration.
As far as the sporadic appearance of heroes, what about linking it to the appearance of a comet. More heroes the closer it passes each time (every dozen years or so), and it either has or will pass the closest it ever has in history in the storyline.
How will local heroes, the government, and big time heroes and villians deal with the knowledge that the comet is coming and there could or will be an explosion of new heroes and villians on the horizon?
Keia