In the real MLB you get an extra man in double headers.
It would be nice if this functionality could get added to OOTP. As well as letting us choose "preferred" candidates for coming up for that day. (Which could change depending on how rested they are) (Needed because of people who don't daily sim and for online leagues.)
When a double header is scheduled, we should be able to even choose a SP from the minors to rest and be ready to come up that day ahead of time, without having to manually bench the player for X days. In the case of online leagues which often sim one week at a time. Take this example: Sim goes from Monday through Sunday. AAA pitcher is schedule to pitch Monday and Saturday in AAA, but is needed for a Sunday double header in MLB. You select them ahead of time for that, and the AAA manager rests them instead on Saturday so they can pitch on Sunday. Simply benching them for 6 days wouldn't work, as they still need to pitch that Monday.
Typically a SP from AAA is called up because the rotation can't handle an extra start, so OOTP should probably default to picking from fully rested AAA pitchers who can start.
Under rotation settings:
https://imgur.com/sshuTx0
You could add something that says "Double header call up" and choose from Best rested SP in minor leagues, best RP in minor leagues (>45% rest), rest best MiL SP ahead of time (which would just use the 1st option if a double header gets scheduled for the very next day or too soon for said SP to get rested), and maybe a "none" while having a place to select specific players to call up under strategy (some may want to keep rotation order and leave on 4-day rest, while calling up some fielder for some reason?? idk if anyone would actually call up something other than a pitcher. I guess maybe a 2nd catcher if running on just one when fielder stamina is set to high)