In the beginning, the Wizard as his younger self had an affair with Elphaba's mother. He had her drink the green elixer - presumably some love potion - and the result was she died giving birth to a green daughter.
Later on in the show, the Wizard had never known this. He offers the green elixer to Elphaba, presumably a potion to...get her drunk.
There aren't two bottles, its the same.
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle