There's lots of software (and hardware options) to manage how different windows are displayed that have nothing to do with keyboard input at all, much less keyboard broadcasting. And then there's lots of different options to provide input to all those different windows. None of this is as simple and cheap as doing it in software, but the community here is working on several different approaches. I think all the different options are still in their infancy and going through rapid innovation and improvement as different people figure ways to improve different aspects of it. We may eventually get to a point where someone can say, "click here and follow the guide". But for now its mostly a matter of building your own solution.

It kinda depends what you want to priorities in terms of performance and what you want to spend to set it up, but I'd suggest reading through these threads if you haven't already.

https://www.dual-boxing.com/threads/...or-multiboxing
https://www.dual-boxing.com/threads/...re-multiboxing