You're already sharing your office. You're not about to give up your classes. "If it's all the same to you, I'd like to teach both classes. I've been preparing those two classes, and I'd be disappointed to waste that effort."
Dr. Ellison's head shake tells you that she's not happy with your proposal. "Bob has many years of teaching experience, and with his return, I intend to take advantage of that experience."
"Aha!" Blankenship brandishes a sheaf of papers in his hand. "I wondered where this proof of the Ulster Conjecture had gotten off to!"
Dr. Ellison sighs. "Bob. The subject at hand is what classes you will teach."
"Yes, right, sorry. What if we co-taught both classes? I feel a certain obligation to young ., what with my return having put him in this spot."
Dr. Ellison mulls the suggestion over. "Co-teaching a class can be more work than simply teaching it, but that would be an elegant solution to the current conundrum."