Replace a single wall swich (controls light and fan) with a decora dual switch
You don't have enough wires for a double switch
You don't have enough wires in the wall for a double switch, unfortunately.  You'd need a /3 cable (black, red, white, bare) for that, but you only have a /2 cable (black, white, bare), and all the wires in that cable are already spoken for.
Maestro to the rescue!
However, Lutron makes a solution for this problem in their Maestro product line; namely, a fan/light controller that uses powerline signaling to communicate with a canopy receiver module.  You'll want to get a MA-LFQM or MA-LFQHW in the appropriate color (WH for white or LA for light almond, for instance), and install it according to the included instructions.