1) Create the Double room and set the quantity of these.
2) Create the Single room and set the quantity of these.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.
3) Create a third "virtual" Single room and set the quantity to the maximum number of singles you can theoretically sell, in this case it is the quantity of doubles plus the quantity of singles you have available.