Hi Mrmo,
For the time-being you will probably find it easier to work on the overlay first, ie. a black outline, then move to the underlay sprite and fill in the previously drawn shape (where both will be visible in the editor).
In the current system, sprites can only "have" overlays, so both sprites will only be visible when working on the "underlay".
Thanks for raising the issue though, I will take a good look at it and see if there is an improvement to be made.