By default, each Appointment has two properties: ResourceId and ResourceIds. Usually, the ResourceId is used when you have only a single resource for the appointment. In case you have multiple resources for an appointment the ResourceIds collection is used. However, the current implementation of the EditAppointmentDialog uses a RadDropDownList for the resource selection. Thus, the user is not allowed to create an appointment and assign two resources to it. It can be achieved only programmatically. This is a common scenario when creating a meeting and you have at least two participants. A possible solution would be to replace the resources RadDropDownList with a RadCheckedDropDownList. Thus, the user will be able to select multiple resources.