Each twinspark engine has two spark plugs for each cylinder, total eight. One is smaller than the other (same on newer engines??), basically the second plug ensures that all the fuel is burnt properly to safegaurd the cat and ensure efficient combustion. It use to be said that this produced more power but current thinking is that its more about a cleaner engine.
Expensive at service time tho...........8 platinum spark plugs!!
Other benefit is it sounds good when explaining to ya mates wink