I'm not sure double headers are all that fan friendly. i don't know folks who can afford 6 hours all that often. I think they worked in the days when crowds were more sparse, fans more hard core, and games were shorter. I don't see a kid sitting through both games.
I'd assume they wouldn't be single admission events. But, honestly, there aren't many people who have time for 162 games, either. Most fans catch an inning here or there, watch the big games and keep tabs on the team a couple of days a week. If they're serious about shortening the season (and I really haven't seen any indications that they are) this shaves roughly two weeks off it.