In fact, it is very easy to do, I have just inherited my class Pen in two subclasses and changed the GenericRelation parameter, sorry for the disagreement :
class BallPointPen(Pen):
pass
class FountainPen(Pen):
pass
class PencilCase(models.Model)
ballpoint_pens = generic.GenericRelation(BallPointPen, related_name="ballpointpencil+")
fountain_pens = generic.GenericRelation(FountainPen, related_name="fountainpencil+")
And it works like a charm.