You have to connect them in serie. Non polarized caps are simply two polarized in serie. An elctrolitic is a resistor (a BAD resistor) if you connect it inverse. It's better if you use a paper & oil capacitor.
Bye
Aiace
Connect them back to back (minus) to get half the capacitance, but it is advisable to ground that connection with a 100k resistor for a typical audio application.