Two set of vocal chords?

Yeah we have two sets of vocal chords they vibrate together to make sound.
when you sing, normally you use only one specific set of vocal chords while the others lay dormant.
the females usually use the front set and males after puberty use the back set.
The way to use both is to get into doing voice exercises.
Go down and up scales.
Get a piano and sing higher or lower than you are used to normally singing(little by little though, sing a lot more notes that you are used to at first, or else you can strain the chords.)