Hi Pilou,

re:
> The fact that extrude works for both in the same time only when on face is flipped...

It doesn't do that.

Flip can flip the direction of selected curves or a surface or entire joined surface.

It doesn't do anything on a face selection.

Extrude does work on this if one or both faces are selected, without any flipping.

However extruding a face selection also tries to do a boolean with the extrusion and that won't work very well if the object isn't a solid. You can turn off the boolean by setting the "Keep separate" option.

So select one or both faces, use "Set dir" and set "Keep separate", should work - no flipping needed.

- Michael