... if you know how to use virtual desktops and shortcuts. You can't look at two screens at a time, anyways.
One use case I can understand is having a 2nd monitor for checking stock prices or checking for a certain event. Other than that, I don't see how it contributes to "productivity" while working or coding for example.
P.S: Tiling WM users may understand this post more
Once I went double monitor, I can't see myself falling back to a single monitor. I've used a single monitor for a good decade and a half before toying with double monitors. I just like being able to have things on one monitor and other things on another. It doesn't necessarily have to be a programmer-specific thing. I now reflect back and hated the idea of having to manage and check so many things while under one monitor. Things would be blinking, but oh wait, I'm gaming, oh wait, I'm browsing too and it's just too much.