5) Set the Virtual Single to be dependent on first the Single and second the Double using the LOGIC SETTING "Sum of all bookings".
6) Set the Virtual Single to "Assign Bookings" = First Available Dependency Room
7) Hide the Single room from the booking page by setting SELL PRIORITY to "Hide". If you are hiding rooms you will need to adjust your overbooking protection in SETTINGS->PROPERTIES->ROOMS ->SETUP. For instructions please see the context help next to the setting.