When I was a boy, I used to sit in Fenway Park and watch a double-header in about four hours. That was before the "twi-light" doubleheader nonsense where they kicked you out of the park after game one and made you pay to get back in for game 2.
Starting pitchers would normally go 7 or 8 innings on a regular basis with one reliever coming in. There were also many complete games.
Sam, those are called separate admission "day-night" doubleheaders. Done mostly for make-ups of rainouts. Most home teams make these up as part of traditional doubleheaders, either afternoon games starting around 1PM, or "twi-nighters", which start around 5PM.
The Red Sox seem to have a deal to do make-ups as day/nighters. Probably because they tend to get about 90%+ attendance at every game, so making up a game as a traditional DH would force them to lose a lot of ticket revenue. Of course, in places like Oakland, where the average house is about 30-40% of capacity, combining the crowds isn't a big deal.