A couple more ways.
PaneSelector, with a blank Row:
Manipulate[{x, y},
PaneSelector[{True -> Control[{x, 0, 1}], False -> Row[{}]},
Dynamic[visible != "only y-slider visible"]],
PaneSelector[{True -> Control[{y, 0, 1}], False -> Row[{}]},
Dynamic[visible != "only x-slider visible"]],
{visible, {"only x-slider visible", "only y-slider visible",
"both visible"}, ControlType -> PopupMenu}]
Dynamic switch between Identity & Invisible:
Manipulate[{x, y},
Dynamic[If[visible != "only y-slider visible", Identity, Invisible]@
Control[{x, 0, 1}]],
Dynamic[If[visible != "only x-slider visible", Identity, Invisible]@
Control[{y, 0, 1}]],
{visible, {"only x-slider visible", "only y-slider visible",
"both visible"}, ControlType -> PopupMenu}]

DynamicWrapper[Dynamic@If[]]I see nowhere before. Thanks for your help. Is there any other possibility to solve that problem? I can use x only as a normal slider, but can't open this slider with the plus... – Manu Jan 08 '14 at 17:07Invisible@Control[]:Manipulate[{}, Dynamic@If[visible != "hide x-slider", Column[{Control[{x, 0, 1}], Control[{y, 0, 1}]}], Control[{y, 0, 1}]], {visible, {"hide x-slider", "show x-slider"}, ControlType -> PopupMenu}]– Manu Jan 08 '14 at 18:13