Problem editiing multiple Shape Key

While trying to define two shapekeys for a bezier I came across a problem (at least for me).
Creating the second shape key is only possible when starting on base of the shape of the object when
the first shape key is displayed in edit mode the way ot looks like when its value is set to 1 (instead of 0).
But as soon as I start Edit Mode, the curve jumps back to it basis form.

Is there a way to circumvent this behaviour?

Cheers!
tuxic