I think I did this before but it died for lack of examples. Basically you have two characters. Maybe you never see one of them, or one's only mentioned in passing, or one of them is always in shadow, or since it's a book it doesn't matter what they look like. They never meet, are never in the same scene and/or never interact with the same characters. Eventually it's revealed that they're actually the same person under different names. Examples:
Hello, Unknown Troper. You'll need to get known to lend a hand here.