Andy M. said:v-chic. I think you got your explanations are a little confused. Bicarbonate of soda is the chemical name for baking soda. It is not what makes baking powder double acting.
Single acting baking powder is a combination of baking soda, cream of tartar and some salt. Another chemical is added to make it double acting.
The first reaction occurs when the powder gets wet and the second occurs when the powder is heated.