Without having Sunday Ticket, under what circumstances would a fan get both doubleheaders? (a 1 and 4 game on Fox and CBS)
You won’t.
Fox and CBS alternate weekends on which network gets the “doubleheader” game (and that game usually starts a little later; this year, it looks like the doubleheader game starts at 4:25 ET).
“Late” games that aren’t “doubleheader” games are almost always games being played on the West Coast. If your local station is running the late (4:05 start) game, but that network doesn’t have the “doubleheader” for that weekend, that station won’t show an early game.
For example, Fox had the doubleheader this weekend (and, in most of the country, ran the GB / SF game as the doubleheader). CBS actually didn’t have any 4pm start games this week, so there wasn’t a conflict…but if CBS had been carrying a home game for Oakland or San Diego today (which would have started at 4:05 ET / 1:05 Pacific), CBS stations which showed those games wouldn’t have been able to show a 1pm ET game, as well.
In addition, if you’re located in the local market for an NFL team, when that team’s game is televised locally on Sunday afternoon, the other network is not allowed to televise a game in your market during that time slot, even though there are games which could be televised. For example, the CBS station here in Chicago ran the Bears / Colts game in the early slot today. Despite the fact that Fox had four early games running today (and had the doubleheader today), the Fox station here in Chicago couldn’t show a game in the early slot.
The exception is if you fall within two different market areas. I grew up about halfway between Washington and Baltimore, so we were in both the Redskins and Colts (and later Redskins and Ravens) markets. We had two different Fox and CBS affiliates, so we had up to four different games to choose from. They didn’t often carry the same game across markets.
I’m not sure where else that would happen. New York, I assume… maybe southern Florida with Tampa and Miami? Cincy and Indy?