The HubSpot integration can only be connected to one GoToWebinar account. It's mentioned in this KB article that "you can use the Zapier integration to set up a simple integration with a second GoToWebinar account". For Zoom, it's a similar situation.
You cannot connect multiple Zoom or GoToWebinar accounts to one HubSpot account, but multiple users in a single corporate account can use the integration once it is connected.
On a single account, it is not possible to organize a webinar in the same time slot. This is a problem for large corporations where you have many countries and many marketers.
Please add the ability to add multiple accounts on GoToWebinar, Zoom or other webinar solutions.