In metal hi-hats are usually played slightly open for a trashy sound with some players even setting the clutch quite low so as to play the double pedal without affecting the hi-hat sound. However this will compromise the open and closed sound from the hi-hats, which is where X-hats, free standing hi-hats usually on the opposite side of the kit, come in.
They enable the drummer to set regular hi-hats more open and mean the left hand has access to the rest of the kit, whether playing double kick drums or not.
Finally, be sure not to clamp the top hi-hat cymbal too tightly in the clutch as it will need to be free to move. Failure to do this can damage the top hi-hat cymbal, causing it to crack in the worst case.