It's called a split flange design. The advantages are having separate pipes for the turbbine wheel, and wastegate. Exhaust flow is forced out faster, and less turbulent. Having a single pipe design forces the flow into a 2 1/2" or 3" depending on size DP, and causes turbulance and flow to disrupt. Spool up is increased by 500 RPM sooner, which means less lag and a bigger power band. :)