The Amateur League of St. Louis intends experimenting this afternoon, their season having opened to-day. Two games are to be played for the State championship, at Grand Avenue Park in the afternoon. The Grand Avenues and Haymakers open the ball at 2 o'clock, and when they get through the Atlantics and Flyaways are to tackle each other. One admission ticket entitles the purchaser to view both games. The only question to be decided is whether there is sufficient time to play two games on the same ground in one afternoon. If it will work the system will be continued throughout the season.-St. Louis Globe-Democrat, April 1, 1877
This is actually rather interesting because, according to Peter Morris, doubleheaders were not particularly common during this era:
More than one game was sometimes played on a day in the early days of baseball, but this practice had largely ended before the advent of professional baseball. There were two good reasons for this. First, owners saw no reason to give away twice as much of their product for the regular price. Second, the absence of lights meant that play would have to begin very early to be certain of completing two games.
The Resolutes and Boston did play a planned doubleheader on July 4, 1873, with separate admissions being charged...Twinbills were occasionally staged in the ensuing years, usually out of necessity when a canceled game had to be rescheduled.
Morris goes on to describe how the doubleheader became more common in the late 1880's and the single admission doubleheader gained popularity around the turn of the century.
But what we have here is an experiment with regularly scheduled, single admission doubleheaders in St. Louis in 1877, twenty odd years before such a thing became an accepted practice. It's unknown at this time if the experiment succeeded and the Amateur League continued with the practice but I'm going to keep an eye out for anymore doubleheaders in 1877 and see what I find.