show dock permanently on the primary display only or simultaneously on both displays
You can achieve the first, lock it to a single display - though it will entail a change in working practice.  

System Prefs > Mission Control
Uncheck "Displays have separate Spaces"

Your Spaces/Desktops will then always move as a pair, locked together.



Then use System prefs > Displays > Arrangement
You can drag the menu bar representation to define your primary display. The Dock will remain always on this display.



As the Dock is always shown either on the active Space/Desktop in single-screen, or only on the primary display of a multi-screen setup, there's no way to make it appear on two displays simultaneously.