I don't know if you know what djing is about the kids are doing a two by two set. Sara is playing the song (constant beat) and scratching on the other and the boy is  playing off one turntable that has a built in crossfader (dj qbert's specially designed QFO turntable) all the equipment is hooked up to the one amp/monitor which is why it's called a two by two set (2 dj's and normally 4 turntables).