Related Questions:
Assign a Booking to two different Staff members
Have two people work a Booking together
Get notified via email to different users
Answer:
At this point in time it is not possible to have multiple assignees to a Booking.
Workaround:
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.
Go to Settings
Under the heading Fields; click on Custom Fields
Choose the sub-heading Booking
Click + New Custom Field
Fill in labels; and make sure the 'Field Type,' chosen is Picklist.
Add the options to the Picklist
Ensure to include a Merge Field Name if you plan on adding this information to Documents/ Emails.
Click Create
There will now be a Field in the Booking for you to add in a Second Assignee.
It is recommend for each User in the Picklist to Save a View that includes the filter to show if they are the second assignee on a Booking.