I don't think you would have much difference in evaporation, but you would have slightly more water. Although I would like having two ponds and will be building a smaller one in the near future, for CC.
My smaller pond averages 3-4 feet when full. By the end of summer, it will lose and another 1-2 feet. By combining the two ponds, the water volume loss is spread over the entire pond, therefore I do not believe it will recede as much. And if I am not mistaking, the deeper a pond is, the less the sun can heat the water thus reducing the amount of evaporation.
