It would be really helpful if midway through a sandbox mission, ideally both during engagements and during maneuvering, you could actually switch to control both sides. It would be really helpful during spacecraft engagement so you could actually test new strategies for difficult scenarios. (say you want to see how a spacecrafts weapons do at simultaneously attacking the enemy fleet while dealing with an incoming missile barrage) You could really explore new tactical doctrine, and best of all it would allow for a rudimentary sort of hotseat multiplayer. There doesn't tend to be very much in the way of information asymmetry during CDE combat anyway, and the vast majority of battles are automated, you just have the other person look away while you are planning maneuvers and hide your remaining delta-v and you pretty much have semi-turn based combat where when a player wants to do a maneuver you just pause the game, switch sides, give your orders, and resume.
The biggest danger in this capability (outside of the coding challenges of which I am blissfully ignorant) is that after not too long players would probably ask for the ability to include the ability to put more then 2 sides in a combat scenario, which I imagine would be a lot more trouble then it is worth. But maybe an "observer" mode that doesn't include views of things like Delta-v or upcoming maneuvering nodes, would be useful. Although I am sure there is already a hotkey to hide a lot of that.