A two circle fight is a nose to tail fight. It favors the aircraft with the better turn rate.
If a two circle fight has adjacent turn circles the aircraft may meet nose to nose.
Chasing somebody around a flat turn is a two circle fight. A rolling scissors is a two circle fight.
A one circle fight is a nose to nose fight. It favors the aircraft with the smaller turn radius.
A flat scissors is a one circle fight even though you never complete a circle.