I'm about to submit some talks, and I'm wondering what should I do if I have multiple title ideas for the same talk with the same description? Submit one talk per title idea, or write the other title ideas in the notes?

I'm speculating, but imagining it from the event organizer's perspective, I think it'd be better for you to put alternate titles in the notes, otherwise they have to keep track of which of your submissions are actually just alternate titles for other submissions.

Yes. Please put alternative titles in the talk notes.