Installing a dual switch from a single switch which powers two wall sconces
I think you misunderstand the design of the switch in the circuit.

A single pole switch in a circuit just makes or breaks a single wire. When it is off, one of the wires is a hot feed from the main panel and the other goes to the light. When you turn it on, it connects the two together and acts as if the wire was one continuous wire. That is why there is no white wire on the switch. Switching the white wire (neutral) is prohibited. 

If you want to control the ceiling fan with a separate switch you will at least need one more wire from the switch location to sconce and on to the ceiling fan, along with the white from the switch location or sconce. (Or a wireless electronic switch could be used with a receiver at the fan location.)

Either way you need more than what you have available.