You can keep them together nymphs being eaten is no more of a threat then if you were to have the species separate. But separating the newborns is always a good idea for many reasons. However just remember that dubia's can not climb smooth surfaces and hisser can very easily. And the small babies are very talented escape artists.
Demon A, we personally have kept hissers with dubias. Hissers will prey upon dubia nymphs. Where does your knowledge of them not doing so come from?