You want to get twice as much done in a set amount of time
You want to work for twice as much time to get twice as much done.
The second one would mean at least 80 hour weeks or worse, so I'll assume you mean the first.
The solution to it is quite simple: stop doing half of the things you're doing right now.
We all tend to waste time on things that don't add value to what we want to be working on, and when we do this "busy work" we hurt our productivity significantly.
Ruthlessly cut out anything that isn't contributing to your broader goals and focus intensely on what actually matters, and you'll do much more than double your productivity.