If there are lights and switches in the bathroom, do you still use a single cable for exhaust fans?

No, you don't.

It is recommended that you change to a double panel to control the lights and exhaust fans separately. You don't have to re-run the line. Your current switch and closure after the lamp and exhaust fan can work normally, it means that these two appliances are connected in parallel, there must be two live wires connected, the two live wires are connected to the two contacts of the double opening, and the two electrical appliances can also share the zero wire.

If you can't pick it up yourself, you can hire a professional to do it. It's simple.

The main function of the exhaust fan in the bathroom is to discharge the moisture and turbidity in the bathroom, because the replacement of air needs to have new air at the same time, generally speaking, the exhaust fan needs to consider the air intake when exhausting, that is to say, when the exhaust fan is opened, it is best to be able to open the window, and the exhaust of the exhaust fan depends on convection.

The switch position of the main light in the bathroom is generally not equipped with a switch because its installation position is basically higher than the height within reach. The switches are mostly set in a position that is convenient to control and conforms to the overall layout.

You don't need it, just connect it to the original line, and you don't need to take a single line.