There a few different possibilities when it comes to creating overlapping appointments. Contact support first to find a solution that would work best for your situation. Below are some ways to how you can get that started:
Allowing Double Booking
You can allow double booking, triple booking, or more with any paid plan.
If you have one calendar, the feature will be in Business Settings > Availability under the Settings tab.
If you have two or more calendars, you can have different settings for each calendar. In Business Settings > Availability & Calendars, click the settings tab next to the calendar you're interested in.
If you have Availability by Type set up, this feature will be governed separately for each group of appointments. Go to Business Settings > Availability & Calendars and find the group of appointments that you're interested in. Click the Limits button to view/change the setting.
Allowing Partially Overlapping Appointments
Partially overlapping appointments can be set up within the appointment type. For example, if you're an acupuncturist and only need 20 minutes to work with a patient at the beginning of their 60 minute appointment, you can allow double booking for the last 40min. To set up padding, click the "Block off extra time before or after.." link, and you can add padding to the beginning or end of your appointment there.
Within the appointment types, set the padding after the appointment to a negative number to allow overlapping. In this example set the padding after the appointment to -40 minutes.
That will allow for an new appointment to be booked 20 minutes into a 60 minute appointment.