Camels do not store water in their humps, as it is commonly believed. The humps are actually reservoirs of fatty tissue. Concentrating body fat in their humps minimizes heat-trapping insulation throughout the rest of their body, which may be an adaptation to living in hot climates.
Although they don't hold water in their humps, the kind of fat that is in their humps is packed with hydrogen, thus all they need to do is breath in air (oxygen) and their body and make the hydrogen in their humps back into water (H2O). So yes, if they have two humps, they can carry more Hydrogen, which means carrying more water. Hope this helps clear up the confusion.