Swapping opponents might give Willow and Angel enough of an edge to win the day. Angelus, as far as we know, hasn't had very much, if any, experience dealing with practitioners of magic. She could set up some sort of ambush for Angelus and take him out from a distance, before he could get close enough to harm her. While that's going on, Angel could try and distract Dark Willow until Willow could show up, then they could take Angelus on as a team.
Notice, I said "might." It would all depend on Willow taking Angelus out before Dark Willow dusted Angel. If Willow could take Angelus out quickly enough to be able to come to Angel's aid, she might be able to take out Dark Willow while Dark Willow was still concentrating on Angel and before she even realized that Willow was attacking.
Once again, the key element here is "might."