Flow Target surface issue

 From:  Frenchy Pilou (PILOU)
11471.5 
I believe that a cool trick will be this one! ;)

-- Take the all external perimeter of the 2 curvated half parts
-- Join it by prudence
-- Project it on a flat plane (Construct / Curve / Project )
-- Make plane of this projection
= Flat surface under the volumes to Flow
-- Make the Flow as usual! ;)
-- And move the full result anywhere you want! :)

EDITED: 14 Jul 2024 by PILOU