Double steaming is a Chinese cooking technique to prepare delicate food such as bird nests, shark fins etc. The food is covered with water and put in a covered ceramic jar. The jar is then steamed for several hours. This technique ensures there is no loss of liquid or moisture (its essences) from the food being cooked, hence it is often used with expensive ingredients like Chinese herbal medicines.
Compared to Double boiler, the concept is the same but the utensils may be different.