Absolutely, you can do that! We allow providers using Fons the ability to "double book" themselves in case they need that added flexibility in their calendars.
However, keep in mind that clients cannot override your office hours and request "double booked" appointments. Providers will have to initiate those invitations.
Remember, you can also set up Group Appointments if your intent is to hold an appointment or class with multiple clients at once.
This is one of those awesome examples of a feature we've built based on the large amount of requests we've had from our users. We love and support you all, so we take your feedback to heart!