Select both Gross Areas / Hall Boundaries and a new dynamic menu ribbon will appear at the top of your screen. From this ribbon click on 'Combine Halls'
The following box will appear asking you which Hall Boundary you would like to keep, in this case I will select 'Hall 3', then click OK.
The Hall Boundaries will now be combined as shown below.
Note: You may need to change the Venue Supplied Area if relevant, which you can do by double clicking on the Hall Boundary.