Kevin once described the phenomenon as "The Willam of Two Worlds Theory." Same guy. Two different universes. They trade places over the course of a two-day period.
Originally Scott Mosier wanted to reprise the role in Mallrats but Ethan Suplee cracked Kevin up so much during his audition that Smith said: "It won't make any sense and will just confuse people, but I want Ethan to play Willam. We'll explain it later if anyone asks."
Oh, I know, I've heard the "Willam of Two Worlds" thing before it's still a bit of an oddity. (IIRC, the Mallrats Willam shows up during the "finger-cuffs" explanation/flashback in Chasing Amy.)