gui construction - How to make slider in/visible in Manipulate?

I need your help with making a slider invisible and visible in a Manipulate. Here is my code:

Manipulate[{}, 
  {x, 0, 1}, 
  {y, 0, 1}, 
  {visible, 
    {"only x-slider visible", "only y-slider visible", "both visible"}, 
    ControlType -> PopupMenu}]

The PopupMenu shows how I want it to function.