Do Dual Monitors Improve Productivity?

As for me, the answer is definitely YES. Sometimes, I really need to see a lot of things at the same time. Multiple monitors are a godsend for multitasking. People who multitask will love double monitors. It also decreases the time juggling between many different tabs. 

I found the dual monitors was really helpful in the situation below.

• Writing. When I was writing an article on one screen, I could open the browser on the other screen to get the reference information. 
• Coding. It's a really good experience when I can watch the tutorial documents or videos on one screen, and at the same time practice the coding on the other screen.
• Gaming. Recently, I fell in love with playing Teamfight Tactics, the new mode of League of Legends. There's some vacant time during waiting or at the beginning of the game. These boring time can be filled by watching the live video to learn others' excellent strategy of playing this game.

Of course, multiple monitors setup also has disadvantages. There's an article named "Why I Stopped Using Multiple Monitors", written by Cory House. The very first reason he gave up the multiple monitors is that it can cause distraction. He is the person who really tried multiple monitors, and said it's not for me. So, I think his opinion is convincible for the most time. Indeed,  Humans can only focus on one thing at a time. 
"Two of the most valuable tasks we do on a daily basis require focusing on a single application for extended periods: Reading and writing. Both reading and writing benefit from the focus that comes from being the only thing on the screen. A book with a moving twitter feed in the margin would be a disaster. But many are willingly doing just that. Additional displays often lead to continuously displaying potential distractions. You’re much more likely to keep working if your email, social media, and so on aren’t visible. 
As a developer, I write code on a daily basis. So I often read docs. But I rarely need the docs visible at the same time I’m writing code. I read the relevant doc, then code. My workflow is modal." And I can see both agree and disagree with voices in the comments.

Except for Cory's blog, I can also see it's talked on Reddit- Multiple Monitors: Yes or No? I sum up the main reason why people like or dislike the multiple monitors.

Cons against Multiple Monitors

• Multi-monitors are a more demanding set-up
• More interruptions
• Less focus
• If my email or social media feeds are available at a glance, then I’ll check them constantly. This isn’t just unnecessary, it’s counter-productive. In a world of endless distractions, being able to focus on a single task for an extended period is a seriously valuable skill.
• It's really up to the individual. I used to use multiple monitors, and at first it felt productive, but I realized a couple of years ago I was just fooling myself. I would find myself doing all the work on one monitor, and filling up the 2nd with distractions (Twitter, Youtube, Movies, etc).
• Multiple monitors is annoying cognitive overhead: what windows go on what monitors? You have to waste time maintaining that organization.

Pros against Multiple Monitors

• I am a software developer and after switching to dual monitors it's almost impossible to switch back. Sometimes I'll have 4 IDE's open at once and wish I had even more monitors. And other times I'll only need a single screen but can still keep my task list open on the second monitor.
• I use two monitors and often work on animation projects with references on the other monitor. Really helps! Right now I’m working on a paper for school. I keep the assignment, the rubric, and a data sheet I’m using for my paper on my second monitor to help keep me grounded.
• I love using multiple monitors, particularly if I need to use multiple programs simultaneously, such as having both excel and PowerPoint open or having two excel workbooks on different monitors. It's also useful for writing/research, as you can have your citation manager and/or database on one monitor and your text file on another.
• It makes life a lot easier for tasks & jobs that require you to have multiple large files/apps visible most/all of the time. I'm a programmer so I use one screen for coding and another for viewing reference material, writing notes, & responding to email/chat. I could probably really use the 3rd screen. I don't think I'd want to look at more than 3 screens, though.
• When I did design work I was using two adobe apps and usually 5-6 browser tabs per project and my downloads folder, it's much easier to have all of that displayed across two screens. Now I have a lot of tasks that deal with data entry, audits, and reports and some of those tasks require side-by-side windows, and the others are just easier with side-by-side windows.

Different person has different options. It may be determined by what's your daily work. If you find the dual monitor just display the unnecessary information and make you distracted. A single monitor is better for you. If you find the dual monitor or the multiple monitors really wonderful, you're comfortable with it, and it boosts your productivity and saves you much time. Then, just go for it.