I try your first approach.
For testing reasons i add one Label to "RegionOne" and "RegionTwo"
At runtime i got the following error:
ContentControl's Content property is not empty.
This control is being associated with a region, but the control is already bound to something else.
If you did not explicitly set the control's Content property,
this exception may be caused by a change in the value of the inherited RegionManager attached property.
If both reagions are empty, there is no error
Does that means that i have to add my UserControls at runtime ?