I generally carry two knives all the time, sometimes three.
They're intended to cover all the bases.
First up is a one-hand-opening folder. This one is usually fairly heavy-duty (Benchmade, Spyderco, Emerson, top-end Kershaw or ZT). It takes care of most of the cutting I need to do.
Next in the lineup is a smaller knife, usually a slipjoint. This is for when I have to go from out on the boats/docks back into the office, where my large one-hander might scare some folks. It also stays pretty clean, so it's my food knife as well, making my sandwiches and cutting my hot sopressatta at lunch time.
If I carry a third knife, it's a fixed blade. I don't carry anything as a weapon, so I have no reason to hide it; it's open-carry, on my belt. Usually, I carry one of these in the winter, when I'll be working outside and my hands are either in gloves or numb from the cold, and would have a hard time operating a folder (or getting into my several layers of clothing to produce it in short order).
Make sense?