I've never gotten into programming and stand alone controllers/boards. If you are comfortable with that kind of stuff, perhaps adapting a late model variable vane turbo is within your ability. It will take up much less space and probably give much smoother performance. Also, throttle bodies won't hold up to the heat of exhaust duty. Especially so close to the manifold. If what you meant was controlling the flow of exhaust through each turbo by using a throttle body.
If you really like the idea of multi turbos, there was recently an episode on Jay Leno's Garage YouTube with a Lancia rally car from the 80's. It had , I believe, three turbos with all kinds of plumbing and waste gates to switch between the three of them. Researching that system would be a good start