Here's the answer:
Harlan and Manny are the same person, but in alternate timelines or dimensions. Harlan is the person Manny would have been had the car accident at Grand & Humble never happened and had his adoptive parents never been killed. Though the course of the book, they seem to almost interact, but of course they never do, because they can't!
Get it? Good! Now go read the book again and see all the clues!
Start with the very first paragraph:
"Two faces. Two sides to the same person. That's what Harlan's teaching was getting at. It was so obvious, he couldn't believe everyone else hadn't seen it right from the start. How could they be so blind?"
There's lots more information at the Grand & Humble discussion guide