Do I need to use this in half dosage since it is double action?

No. Double action means that the baking powder acts twice during the process of baking to ensure that product gets good and consistent rise.

Why is SUPER PREMIUM DA2 better than ECO SUPER DA2 Baking powders?

While both the baking powders are double acting, the primary difference is in the strength of the salts used in formulating these products. SUPER PREMIUM DA2 has higher CO 2 release strength as compared to ECO SUPER DA2 and it is also Aluminum free in formulation.

What is the shelf life of this baking powder?

Typically, if the baking powder is stored below 21ºC it has a shelf life of about 12 months from date of manufacture. Its strength will slowly start reducing once the product nears the end of shelf life period.