Curious what the simplest way is to do subject line testing by audience? I have one email that is going to two audiences. I want each of these audiences to get a different subject line; is there a way to do that by just sending the email in one send?
Are your audiences mutually exclusive? If so, you can always run a Smart Campaign with two Email assets (same email, just different subject line) and just use choices on the Send Email action in your Flow. Because you're not explicitly testing two subject lines to the same segment, I'd avoid trying to use an Email Program to get your outcome.