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.
Setup the room dependency settings in SETTINGS->PROPERTIES->ROOMS->DEPENDENCIES as follows:
7) Hide the Single room from the booking page by setting SELL PRIORITY to "Hide".
5) Create a "virtual" Single room and set the quantity to five.
Setup the room dependency settings in SETTINGS->PROPERTIES->ROOMS->DEPENDENCIES as follows:
5) Set all Dependency Levels to Ignore Sub Dependencies.