Found the answer to my own question. The property is "Active" - don't know why I didn't think of it before.
C++ :
Series1->Active = False;
Series2->Active = True;
EDIT : Beware, even if you change the title of your series, the "real" name to use in this code will probably still be Series1/2/... . This is because every Series you create (by right clicking on the TDBChart->Chart->Series->Add) creates a TBarSerie which seemingly can only be accessed from the Object inspector. The title you give it doesn't affect its real name.
Thanks for your comment, Narcis Calvet.