At this point in time it is not possible to have multiple assignees to a Booking. 

Although you are unable to add a second assignee; you can use Custom Fields as a workaround that will allow you to tag a second assignee to a Booking. They will not receive notification emails the same way an Assignee would, but you will be able to add these filters to Views in Bookings, allowing you to create a Custom View for people to easily see if they are also assigned to the Booking.