And no comes a reply to myself !
We figured out how to hack towards the behaviour we need.
1/ we add an ObjectiveC .mm file to the project, to call somme MacOS Cocoa's stuff
2/ At application AppB startup, from this ObjectiveC file we call[NSApp hide:nil]
which "hides" the application, and reactivate our AppA.
Well... This turns into some flickering of the active application (because AppA looses for a few ms activation when AppB starts ; then AppB hides itself and AppA gets the active app again), but we can live with that !