With HeySummit specifically, this is unlimited. It is all dependent on the webinar platform you use to host your live talk.
If you have an account tier that allows for concurrent talks, then it will work within HeySummit. If you don't, it will not work. For example, Zoom will not allow a host to hold concurrent talks, but you can get around this by adding multiple hosts on one account (if your account tier allows you to do so), and having them host different talks at the same time.