From EXACT version 11.3, it is possible to roster two Providers in a single room, so long as they are different Provider Types (Dentist and Hygienist).
To create a Roster with multiple Providers
Create a Roster with one Provider.
Repeat the procedure - drag over the same area of the Appointment Book and add a second Provider, ensuring that the second Provider is a different Provider Type to the first.
The roster changes outline colour and is now labelled with both Provider names:
Mouseover the Roster and check that the tooltip reflects the correct information: