Unfortunately, this is one of the known limitation of Marketo native integration with webinars: it's 1 Marketo program for 1 webinar session.
There is not much to be done around it.
In this situation, we usually have an umbrella program that manages registrations with a picklist or radio button for the visitors to choose their session. This umbrella program is not linked to the webinar platform. Then, when someone registers, the umbrella program adds the person to one of the "session" programs that are linked to the webinar platform.