I presume that you are referring to the option on page 56 (which applies to a single weapon), not the ones (Add./Sec. Attack) on page 58 (which basically defines Two Weapon Combo).
With this option, the character is able to make 1 attack roll for every 10 ranks. For each attack beyond the first, ALL attacks receive a -20 modifier.
If the character decides to parry, the amount subtracted from OB and added to DB would be subtracted from his OB, and THEN his OB would be adjusted according to the "Additional Attacks" option.