Study the guitar work of Metallica. When listening to, specifically, the guitar sound of any band (not the song overall), the listener gains a new perspective. This is even true of someone who is already a Metallica fan.
Watch DVDs and videos of Metallica performing. Look at the guitar playing. Make note of both the playing of lead guitarist Kirk Hammett and rhythm guitarist James Hetfield. The interaction between the two can give you ideas on how to assimilate both styles when playing a Metallica song on one guitar.
Take out a Metallica songbook from your local library or look for legitimate sources of their songs and music online. Learn the chords of Metallica's songs.
Record your own playing. Listen for any weak parts and work on these.