Hosting Back to Back Meeting and Webinar

Question: Why can't a meeting and webinar run simultaneously when an alternative host is assigned?

Longer description: I am the admin for an organization account. One of our account owners scheduled a meeting that overlapped with a webinar. They assigned an alternative host to start the webinar. The alternative host is a Pro user and has their own webinar license. When the alternative host went to start the webinar, they received a message that a webinar couldn't begin until the active meeting ended. I've read that an account can have two meetings simultaneously as long there is an alternative host for the second.