Maybe this is the solution for the "advanced scouting" research after chapter 18. Add a research that gives you another scout. People who don't want to wait that long could buy it sooner with diamonds, maybe.
I'm not sure how much coding it would take to accomplish this. There appear to be some contingencies in the game that do not occur until a province has finished scouting, in particular the cost to scout remaining provinces. I think that increase occurs after the previous scout has started, though, so it would not be a problem to have two scouts going at once. Other features like the remaining time to scout and the navigate to current scout functions would have to be altered to allow for a choice between two scouts.