There is a special scenario in PRO that allows you to set up doubleheaders, where the same teams play each other twice (or 3 times) in one day.
You will need to set your number of games to a multiple of the number of weeks, e.g. 20 games in 10 weeks.
if that condition is met, you will see two extra options under Allocation Options:
Force Same Day Doubleheaders
Additionally, you may want to set the max wait time to slightly above 18 hours, in order to force the doubleheaders to be played on the same day (18 hours is the threshold, where it switches from only applying the max wait time to games on the same day to checking across days).