Here is a great article:
Twin Scroll Turbo System Design - Modified Magazine
It compares the open housing to a twin scroll housing...but you can see the advantages of the twin scroll here.
With a twin turbo system, you do not get the crucial scavengeing effect that makes the twin scroll housing that much more efficient. This same priciples apply when you are comparing the twin turbo log style turbo manifolds to the twin scroll system with equal length primaries. With a log style manifold there is a lot of interfeerence between the exhaust pulses in each bank because the runners are all different lengths. There is no scavenging. Now, if you had a twin turbo system mounted to equal length primary headers, you would have the same scavenging effect as a twin scroll kit with equal lenth runners. This would be ultimate setup for high, efficienty power levels.
Like this one:
The setup above is a twin turbo kit, but with equal length headers. This is very important when it comes to the scavenging effect. OEM headers on the 370z are equal length, and this is why the twin scroll kit works great.