Baking soda plus cream of tartar is
a single action baking powder. It leavens or create bubbles only once, as soon as the liquid is added.
A double action baking powder has two leavening events and consists of baking soda plus two dry acids. The addition of liquid starts the first action and addition of heat starts the second action. The second dry acidic agent is commonly either sodium aluminum sulfate or calcium acid phosphate.
You can read more about baking soda and baking powders and the chemistry involved here: