Sometimes both are needed as supplements… As in the case of the interaction of Jacob and Leah.
…sometimes only one. As in the case of Israel (=Zeir Anpin) and Leah or of Jacob and Rachel. (The main Partzufim through which Zeir and Nukva are manifested are Israel and Rachel, but sometimes two others come in their place – Jacob and Leah. Sometimes both come, sometimes only one, such as Israel and Leah or Jacob and Rachel.)
The underlying foundation of all this is that the light of Zeir Anpin or the Nukva also shines through Jacob or Leah, in which case the latter supplement Zeir and Nukva and become aspects of them. (Earlier it was explained how the Partzufim of Jacob and Leah were instituted – through the repair of the fallen hind parts of Abba and Imma. Now we see that the interaction of Jacob and Leah may take the place of that of Zeir and Nukva while the latter radiate through them.)