I think you have a couple options here:
If you have enough budget: just as Nate said, look for a automated dolly, it's definitely the easiest way to get it. As someone who has ALWAYS done 3 characters or more in all his productions, this thing would have been a life saver!
If you have a chroma: use it. Unless you like pain and struggle and want to go through hours of rotoscoping, using a green screen will make it way faster in postproduction time. I recommend to shot all static and then add digital movement, although obviously it's not going to look as great as if you had one of these robot heads.
If you don't have any of the above: GET CREATIVE. The Austin Powers trilogy probably is a production you want to check (again, if you have already). Mike Myers does a couple of characters, like both the main character and the main antagonist, and they use doubles, over-the-shoulder shots, and some other techniques that helps selling the effect.
In this video, I play Raccoon (the guy with the gas mask) and Bubbles (the guy with the Monkey mask). All my projects are zero-budget, and in this particular scene I didn't use any green screen nor fancy gear, I hope you find something that helps you get creative!