As a human you can only focus your eyes at one single thing at a time.

Whether you have to move your head/eyes or press a key to switch between the documentation and the IDE does not make much difference in term of efficiency.

I actually find it more comfortable, ergonomic and fast to have a single screen and switch the workspace. That also means that my (physical) setup is minimal and easy to manage, and that I do not get unnecessary distractions or things moving (notifications, messages, ads, animations...) in my peripheral vision.