Twin plugs are usually used for the reason Sport_Pilot touched on, two seperate ignitions for a wider flame front. The idea being that, in larger engines having a wide combustion chamber, the flame front is more evenly spread because each one has less distance to travel to spread across the piston area. Pressure rise is then faster so the ignition point can be set closer to TDC to make better use of the pressure where it's needed after TDC. Any pressure rise before TDC works against crank rotation so it's wasted power. Twin plugs are most effective in large, high speed engines where there isn't much time for the pressure rise to happen.