I created a Multiple Attack Proficiency (MAP) skill for my games. Without going into too much detail, the skill works by off-setting the penalty of all fo the OB bonuses that a character may have (without the skill, no bonuses other than rank are applied to the OB when making mulitple attacks).
A character can make one attack with 10 ranks (uses those ranks as the OB bonus) and the other (multiple attacks) with each increment above the 10 ranks.
Example: a character with 12 ranks and a total OB bonus (excluding rank bonuses, but including stats, magoical items, profession bonus etc of +50) of +54 (10 ranks at +5 and 2 ranks of +2)
This example character (Bob) can attack as follows:
a. Once int he round at +104 OB
b. Twice in the round at +50 OB and +4 OB
...or with 4 ranks in MAP (+20 total MAP bonus - as there are no Prof or stat bonuses for the skill)
c. Twice in the round at + 60 OB (+50 from ranks and +10 taken from 20% (the MAP skill) of his OB bonuses) and +14 OB (+4 from ranks and +10 taken from 20% (the MAP skill) of his OB bonuses).
The skill is developed as the same cost as the chosen weapon and must be specialized in that particular weapon. The skill can only be developed in a weapon in which a character is able to use MAP, i.e., the charater has more than 10 ranks in said weapon skill.
Works fine for us.
As far as attack order, for that particular skill, all attacks are made on the same initiative. So far, I have had no reason not to allow that.