SC1 Vanilla’s final mission is like that, and the bonus campaign that shipped with Allen Schezzar in Broodwar is like that, you can control a Terran base and a Protoss base simultaneously and make 400 supply of tanks and Carriers, but the mission is not really that hard after all, but you have to stop a Torrasque ultralisk and kill a cerebrate to stop it from respawning on you.
If you want to make this mission more challenging, open it i nthe Editor and replace the A.I. scripts with the Insane AI.
Blizzard did do this some in Warcraft 3 as well, where you control Illidan’s Naga and Stormrage’s Night Elves simultaneously.
Yas the Enslavers campaign is fun as hell. The only thing that is missing is voice acting for all the unique character units they give you to use.
Didn’t know you could do that, will have to try to open it that way. Warcraft 3 has a custom campaign with Illidan? Going to have to play that sometime.