While the idea that more desktop real estate means higher productivity seems sound, in practice I found it mostly just invited distraction.
The fact is, I can only look at one monitor at a time, so whether I’m turning my head or using a keyboard shortcut, I’m not ever going to be able to see code and output simultaneously without putting those two windows on the same monitor.
So more often than not I just used the extra monitor to keep email or social media open constantly, inviting those things to distract me or, worse, trigger a stress response via peripheral vision.
In short: Mo monitors, mo problems.