Divided is when the exhaust housing of the turbine has two separate inlets to the turbine wheel. A divided manifold is one that has two separate outlets in one flange.
Thats a divided turbine housing.
Thats a divided manifold.
The reason behind the divided turbine housing and manifold is that a properly designed divided manifold uses alternating pulses of the exhaust based on firing order to time the pulses equally to the turbine.
On our motor cylinders 1,2 and 3 would go to one side of the divider and 4,5 and 6 would go to the other. When the engine is running the pulses would alternate from one side of the divider to the other so the pulses would keep a constant push on the turbine wheel.
It helps with scavenging as one pulse isnt running into the next in the collector and is fully implemented when used with equal length runners.
Performance wise, other then spool i dont believe there is any. Quicker spool could be had with a proper divided system.
Correct any of that if its wrong.