1995. Greg Maddux and Mike Mussina won 19 games apeice and led their respective leagues.
That was a short season, though. When was the last time it happened in a full season?
EDIT: I checked. It has never happened in a full season. The only other times it happened were 1994 (Jimmy Key had 17 for the Yankees, Ken Hill and Greg Maddux each had 16 in the NL) and 1981 (Tom Seaver, Jack Morris, Dennis Martinez, Pete Vukovich and Steve McCatty all had 14).