If anyone needs help with this, basically if two employees have any difference in their schedules then it's impossible to have them share a calendar and be able to be booked for the same time slot (on days when they work together).
If one or more employees have exactly the same schedule, then here's how to make this work:
Create an Appointment Type for each employee.
Create a Resource for each employee (Appointment Type). Set the Quantity field to 1 for each Resource.
On the Availability page (by default it will take you to the Calendars page), make sure both employees are together in the same Group Appointment Type.
Click the Edit Availability/Limits button for the Group. Then, within the popup, click the Calendar Scheduling Limits tab and update the Appointments Per Time Slot to 2.
It'll include a message saying that appointments can be double booked, but the resource limitation of 1 will prevent the same employee from being double booked.
Okay, that's all, hope this helps someone one day!