Heath isn't a second lead in The Dark Knight.
His character doesn't contribute enough to the story to be considered a second lead of the story/movie.
My mind has just blanked on good examples of a second lead. There aren't too many second leads in contemporary films.
Not trying to tell anybody who they can/can't nominate for what.
But this is PTP, not 'Nam.
There are rules.
Peace

"Your current frequencies of understanding outweigh that which has been given for you to understand." Saul Williams