I just started playing Star Legions a couple days ago. It is very unique indeed. I can't think of any game that tried to copy this complete "formula"... that is, being able to scan the surface of a planet to pick your invasion sites (cities), assign each city a "zone", and then assign paratroopers and marines to each of upto 8 zones using dropships; then try to monitor each of those zones simultaneously from orbit, watching the soldiers fight in real-time, and even jumping into the action at zone-level to direct individual soldiers if they get into trouble.
There are games with some similarities to some aspects of Star Legion that I can think of! Microplay's Carrier Command was kind of a sci-fi naval version of this game, where you can command upto 4 different vehicles (amphibious or aircraft) and watch their progress in 4 different monitors simultaneously. I believe the game Hostile Waters, which was briefly sold on GOG, was a modern successor to Carrier Command.
The EA game Space Hulk lets you command upto 4 Space Marines (from the Warhammer 40K games) in a kind of "Aliens vs Marines" game. Although it's a hybrid realtime/stoptime game I think. You can watch all 4 marine's viewcams simultaneously and jump in and control one of them if you want.
But as for monitoring and controlling multiple armies simultaneously? I can't think of another. I'm probably just drawing a blank!