I always had a hard time remembering who was who; was there some distinguishing character traits between Nyssa and Tegan? I think the Fifth Doctor's travels would have been alot better if they had cut one of the two girls.
I believe they wanted to cut Nyssa at one point, but Peter Davison objected on the grounds that he felt she was better suited to his Doctor than Tegan. Ironically, while the Doctor may have seemed to favour Tegan, Davison thought the opposite.