[RPG] Attack Bonus question for Monsters with multiple attacks from multiple weapons

This issue becomes clear once the rules on actions in combat and natural weapons have been carefully studied.

A good way to understand what kind of action each type of attack requires is to think of the actions first. Go to the standard action subheading and observe that it only allows you to make a singular "attack". The "full attack" option, which describes how to attack multiple times in a round, is described under full-round actions. (In short: Standard action = one attack, full-round action = all attacks)

As for the Marilith, the comma separating items in its full attack list is an AND. Thus, the full attack line describes two options:

1. Making its normal number of iterative attacks (stemming from its BAB of +16) with its primary weapon, followed by 5 attacks from its secondary weapons (held in other arms), followed by a single tail slap.
2. Foregoing all of its sword attacks to perform natural slam attacks with all of its appendages, and then a single tail slap.

There is a ruling by James Jacobs in the "Ask James Jacobs ALL your Questions" post the on the paizo messageboards.

James Jacobs Creative Director

1. If a creature dual-wields two Speed weapons, does the creature gain one additional attack total on a full attack, or one attack per weapon of speed on a full attack? [...]

1) No... speed effects, whether they come from multiple speed weapons or multiple haste spells or any combination of the two, do not stack. You can only ever gain 1 additional attack from a haste effect in a round.

With James Jacobs being the single topmost man directly responsible for Pathfinder, this thread is pretty much as official as it gets with Pathfinder rulings, falling just short of the official FAQ.