Hi Dimitri, great result on your first try!

re:
> The unification of the two halves did not happen -despite all my tries- in MoI (I do not know why).

Part of the process of doing booleans is that the objects are intersected with each other to generate intersection curves. If the objects just barely skim over some nearly overlapping surface area it can be difficult to get well formed intersections. It tends to be better to have objects push through each other somewhat rather than just barely skim each other.

Another thing you can do to see why a boolean isn't working well is to generate the intersection curves by themselves by using the Construct > Curve > Isect command. In order to be booleaned those intersection curves should make closed loops. If the intersection result looks all messy and complex then that's the problem.

- Michael