Hi Nacho,
1) Make Part "B" as Phantom Assembly in material master mm02 - mrp 2 - E 50 and create the order of "A". Order will not be created for B automatically A can be produced and B can be consumed.
2) Or dont use scheduling parameters. Make it empty.
3) Or set ROP or demand creation for part A. B order automatically will be created.
Regards
Maddy