If you look closely at the last two posts, you might notice one name that's shared across the lists -- Jaime Moreno. So far he's the only person to lead the league in both assists and goals (albeit not in the same season.)
No player has ever led the league in both categories in the same year, but two players have come close. In 2006 Real Salt Lake's Jeff Cunningham led the league in scoring with 16 goals, and ended tied for second in assists with four other players. One of the players he was tied with was Christian Gomez, who tied Ante Razov for the second most goals at 14.