Actually they live in one of those rich neighborhoods where the families are arranged alphabetically.
Cassie's family, the Harighntons, are from an auld English branch of the Lincolnshire Haringtons (still not the same name as there is only one 'n'). When the Haringtons came over with the first Virginia colonies they added the 'h' and shifted some letters around to make it sound more "new worldly".
Meanwhile, Roger's ancestors were from some shit-hole East of Warwick and didn't come over until after the American Revolution (Patriarch Horus Harrington had a penchant for drawing dirty cartoons of the King and a goat. He found that in America he could freely print whatever he damned well pleased and went on a tear with a series of filthy panels called "King George's Fucking Goat").
So yeah. As you can see, there is clearly no relationship between these two families or their respective names. Glad we could finally put that to rest.
edit: For the record, all this was going to come in the game eventually. Probably post-coitus somewhere.