Seperating bathroom light and exhaust fan on single switch
It is possible that the two black wires (other than that going to the switch) are going separately to the fan and light. If so, the change is easy.

To test this do the following:


turn off the breaker
separate the three black wires nutted together
wirenut the black to the switch with one of the others and insulate the other in a separate wirenut
turn the breaker back on test the switch. Hopefully, the switch will operate only one of the fan or light.
turn the breaker back off
switch the black wires so the other one is connected to the wire to the switch
turn the breaker back on and test the switch. Hopefully, it will only control the other of the light or fan
Turn the breaker back off and optionally restore the original wiring


If the test results followed what I marked "hopefully", you are in business. Remove the other wire to the switch and wirenut it to two wires to run to one side of the dual switch (if one side of the switch has a removable bridge, just connect the single wire to that side). Connect the two blacks originally wirenutted together to the other side of the switches (if that side has a removable bridge, remove it).

For the pigtail wires, you should use the same guage wire as the rest. Examine the wires for printing on the insulation (look for an AWG number).

I'd also recommend against using the back push-in connectors. It's better to use the screw terminals.