I’ve never understood why a player is locked for both games of a doubleheader at the start of the first game. This makes no sense to me. Why can’t I change out a relief pitcher who pitched 2 innings in game one or a catcher who played 9 innings in hot weather? As long as I am replacing a player who played in the first game of a doubleheader with a player who hasn’t started their game yet, I think this should be allowed.
I’m sure there isn’t anything wrong with it from a game point of view, probably just the way it was coded to begin with and not a big enough problem to make it worth spending time to change it.
If your league wanted to allow it, you could allow it. However, none of the big fantasy league hosting sites probably support this, so it’s probably not going to be possible to implement.
There actually is a scheduled doubleheader this year between the D-backs and Rangers on May 27. With the inevitable expansion of the post season, I wouldn’t be surprised to see even more scheduled doubleheaders in future years.
Because most of the fantasy sports sites work on a daily basis. It’s the same reason why you can’t, say, start a catcher who plays in an afternoon game and then another who plays in an evening game the same day.