The difference between S70GL02GS11FHI020 and S70GL02GS11FHI010 is only the protection area, one is high address while the other is low address, if you are not using the protection functions, there are not different between there 2 parts.
GT grade is our highest reliability and quality level specially designed for automobile applications, if you are using in the normal application , the industry level part number is enough.
Another difference between GL and GS is GL using 3.3V Vcc while GS using 1.8V Vcc, the PCB design is different. The dual-die package need 2 CS# pin to select these 2 parts, but one die package only have 1 CS# pin.
The part about the dual-die device needs two #CS/#CE pins in not correct. Externally there is only one #CE pin. Internally the #CE pin is connected to both dies. The die selection is the highest bit of the address bus.