Adding multiple receivers to a Ditto Room is only available for Ditto Premium and Elite plans. Ditto Basic plans can be upgraded in the Billing section of your Ditto Account Portal.
Yes, Ditto supports multi-display mirroring. If you've set up multiple receivers in a Ditto Room, you will have the option to screen mirror to all receivers in the Ditto Room or select which receiver to mirror the content to. You can follow these links to learn how to:
Set up multiple receivers in a Ditto Room
Connect to all receivers or a specific one in a Ditto room.
Note: By default, Ditto will automatically screen mirror to every receiver in that room unless a receiver is selected as the target device.