Make dock appear on both screens at the same time
tell application "System Events"
    tell dock preferences
        properties
    end tell
end tell


This is everything supported natively:

{minimize effect:genie, magnification size:1.0, dock size:0.321428567171, 
autohide:false, animate:true, magnification:false, screen edge:bottom, 
class:dock preferences object}


Plus, as purely a philosophical question, the Dock can not be displayed on both screes at the same time.

Suppose you have one Dock per display. If you have cursor focus on display 1 (i.e. Menu Bar is not greyed/faded out), you move the cursor to display 2 with its greyed out Menu Bar and click on a Dock to launch a new app.

Where should the new window go? It should go to display 1 because display 1 has the focus. Yet it should also go to display because it's where the Dock that launched it is at.

Take a step back, there is another question. What should happen when you click on Dock on an unfocused display? Shouldn't focus be switch to the Dock/display before anything else? So launching an app is a double click process. Or more precisely, single-click if the display is in focus, double-click if not. And how do you tell if the Dock is in Focus? Move your eyes all the way up to Menu Bar (or Window Bar of frontmost window).

This is not an issue on other OS's. This mess can be trace back to when OS X compromised window focus long ago by place Menu Bar on top instead of in window. Not able to have multiple Docks is one small ramification of that design decision.