I am pretty sure that the closest you can get to this without an add on (of which there are probably several) is to show both action bars and set up the bindings on the new bar to be the same buttons on the original plus some modifier. For completeness you show extra action bars under
Interface -> Action Bars* in the menu.
*Can someone please confirm the name of the sub menu, I do not have access to the game currently, if not I will do so myself later.
For example I have one bar key-bound like:
| Q | E | R | T | F | G | 1 | 2 | 3 | 4 | 5 |
and the second bar is identical but using shift:
| S+Q | S+E | S+R | S+T | S+F | S+G | S+1 | S+2 | S+3 | S+4 | S+5 |