The best way to protect your double hung windows with a single wireless contact is by placing the wireless contact in the middle of the window, where the lock is located with one piece on the frame of the top window and the other piece on the frame of the bottom window. This way the sensor will register opened if you open either the top or the bottom pane.
Here is a picture of the installation.
And another picture from the side.
You could also use two separate sensors, one for the top window and one for the bottom, if you do not have room on the middle of the window frame for the type of installation shown in the picture.
If you are going the hardwired route, then you would want to wire 2 contacts (the GRI VS-06 venting switch is a popular choice) in series, one set for the top and one set for the bottom, with your resistor in series after the last device. You might also consider placing two hardwired contacts into the terminal block that is usually located on the inside of the wireless transmitter. This is for experts as it involves wiring.