WPF - Position Popup below each other

Hi,
I got a popup, containing a grid with some dropboxes.
Depending on which dropbox have been selected (changed to) a second popup opens.
That all working fine.

However it seems you can only set a popup's position relative to where the mouse was clicked, causing the new popup to partially be over the first popup.

I need the second to be BELOW the first one (cannot set specific offset's since I don't know in advance how big the first one will be).

Any ideas? I guess my answer will be something between Placement, PlacementMode and PlacementTarget, but could not figure it out yet.