Caldric's Stone Age Double is a Bottom Bar replacement Skin. It incorporates adding one of the Quickslot Bars into the Bottom Bar for a more unique look, showing more than "one" Main Bar on the Bottom Bar graphic.

To do so, once you replace the skin, you want to follow these steps:

1) Open your Main Menu
2) Go to "UI Settings"
3) Go to "Quickslot Bars"
4) Select the Bar you wish to use (1-5)
5) On that Bar, check "Always Show" and make sure it's not docked
6) Add an Ability to that bar (for alignment purposes)
7) Use "Ctrl+\" to unlock your UI
8) Drag the Quickslot Bar into the Bottom Bar graphic and align it
9) Lock your UI and you're ready to go!