The rhinovirus, or common cold, has 99 known strains, which means that it's theoretically possible to catch more than one cold at a time. But it's doubtful that doing so will double your symptoms or make the illness last twice as long.
According to at least one study, however, when you do catch more than one cold at the same time, these two strains of the virus can trade genetic material with each other and create an entirely new strain of cold. Rhinoviruses are always changing (mutating). This is why there is no vaccine against the common cold — and why there probably never will be.
Imagine, if you will, two strains of the virus entering your body, each with its own recipe box. The viruses meet each other, shake hands, trade a few recipes, and move on. A mutation just took place, right inside you.
An old wives' tale used to say that adults catch fewer colds than kids do because once you've had a certain strain of the cold virus, you don't catch it again. According to this logic, as soon as you've had your 99th cold (and worked your way through all of the known strains), you should be immune and never catch a cold again.
Sadly, this is wishful thinking, because the speed at which cold viruses mutate trumps your exposure to each strain. Your immune system is like a doorman that only lets most strains of a virus inside your body once. But when you come in contact with a strain of rhinovirus the second time, its mutations may be significant enough that your immune system won't recognize it from its previous visit. The virus gets in, and next thing you know, you've got a cold.
If adults catch fewer colds than kids, it's based on the same logic your mother is always drilling into you. On average, adults wash their hands more frequently; are less prone to touch their eyes, nose, and mouth; and generally spend less time in close quarters (classrooms, school buses, gyms, daycares) with other people who may be sick.