If you open your vector file in Affinity Designer, select both of the curves then perform an “Expand Stroke” followed by a “Weld” you should get what you want.
See attached Affinity Designer file. Note that I’ve saved the history with this file so you can see exactly what I did. When you download the file you’ll need to remove the “.txt” extension.
Hope this helps and is what you’re looking for.
Feather.afdesign.txt (70.8 KB)