neuroanatomist wrote:
The Dock will be on the main display, not on both displays.
That is not entirely correct. The Dock will only appear in one display at a time, but it can be moved from one display to another, without changing the main display setting.
With two displays side by side, and the Dock positioned at the bottom, the user would need only bump the cursor against the bottom of the second display to make the Dock move.
With other display arrangements, things may vary. JulyIGHOR and Barney have already discussed what could happen with displays set atop one another. And if the displays are side by side, and the Dock is set on one side instead of at the bottom, then it will only stay on the display that is on that side.