Using dock on multiple monitors with the Mac OS X
The Dock has become a really helpful feature for mac users. The Dock is a super comfortable place to save items you use frequently, but is it possible to use it in two or three monitors? The answer is, it’s not really impossible, it’s almost possible.
OS X Yosemite gives us great multi-monitor features, and its ability for the menu bar to appear on all of connected displays. Unfortunately, the Dock can only appear on a single display. On OS X Yosemite and OS X El Capitan, the Dock can appear on any screen regardless of the default display designation. When you’re on a particular display but the Dock is on another display, put your mouse down near the bottom of the screen and the Dock will go to your current display. The dock stay there on the monitor until you restart the computer, or move it to another display by following the same step.
Also you can change the Dock’s default display , go to Apple Menu > System Preferences > Displays > Arrangement. There you can drag the white menu bar that is shown on one of the displays to the display you wish to be designated as the “default” monitor.