This issue is relative of showing the several booking calendars (booking forms) of the same booking resource at the same "visible" page. Its possible to show only one calendar (booking form) of the specific booking resource at one visible page. You will have no problems to show the calendars (booking forms) of the different booking resources at the same page.
At the Booking Calendar Free version is available only one default booking resource, so its mean that any calendars (booking forms) in this version is belong to the same booking resource. And thats why its not possible to show several booking calendars (booking forms) at the same visible page.
This info is relative to the several booking calendars (booking forms) inside of the content of the page or inside of the content of page and visible booking calendar widget (in this case you are need to deactivate the booking calendar widget, or add the exception of showing the widget at this page).