1) In act two, Amalthea has to transform back into the Unicorn almost instantly. I'd prefer to have the actress playing the Unicorn simply step onto the stage than bring the action (which is at a climax at that point) to a screeching halt while a quick change happens.
2) If the same actress plays both, she will have to be on stage for nearly every scene in the play. Splitting the responsibility will keep any single cast member from being overwhelmed.
3) The Unicorn is not going to be represented literally, as I explain in this post. If the same actress plays both of them there isn't enough difference between the Unicorn and Amalthea. I'd like them to have striking differences.
4) Now on to more thematically important territory. As a piece of ensemble story theater, this production of "The Last Unicorn" will have a group of characters playing many different parts. One of the reasons for this is I'm interested in exploring the resonance you get when, for example, the same actor that plays Captain Cully also plays Drinn, or the same actors that play Haggard and Lir also play the two Hunters at the beginning of the story. So, I want the same actress that plays the Princess in that brief Prince/Princess (this scene is only in the book, not the movie- movie fans, read the book! I promise you won't regret it) scene to also play Amalthea. I think it creates a pretty cool resonance- it plays with the idea that Schmendrick's magic sort of "borrows" the appearance of the Princess they see before to transform the Unicorn into a human.
5) It allows us to involve the Unicorn's original form more directly in scenes in act two, including when Amalthea has her nightmare, and when Amalthea uses her secret Unicorn powers to disrupt Mabruk's spell. The most important reason for having two actresses play the Unicorn and Amalthea, however, is it makes the following exchange possible, in the act two Kitchen scene:
(The UNICORN appears and speaks in tandem with AMALTHEA.)
UNICORN/AMALTHEA: The magician gave me only the semblance of a human being. But now I am two:
AMALTHEA: And this other that you call ‘my lady.’
UNICORN/AMALTHEA: She walks in the castle, she sleeps, she dresses herself, she takes her meals,
AMALTHEA: -and she thinks her own thoughts.
UNICORN: And every day she searches the sea and the sky for something she cannot always remember.
AMALTHEA/UNICORN: What is it that she is seeking?
AMALTHEA: (as the UNICORN exits) She knew a moment ago, but she has forgotten.
Pretty neat, huh?