It only applies to Str. So say a fighter has a STR of 18(+4) and a BAB of +5, using 2 +1 Crescent knives one with flame and the other ice, with weapon focus and weapon spec, ambidex and 2 weapon fighting.
the priamry hand would be +9 and the secondary would also be +9 to hit, giving the weilder 4 attacks at +9 (2 for primary and 2 for secondary)
the damage would be
1d3+1(magic)+2(weapon spec)+2(.5 str)+1d6(flame) for the primary
1d3+1(magic)+2(weapon spec)+1(.25 str(becasue secondary hands are halved one already))+1d6 ice
If you GM allows this weapon it makes for a very fun and interesting character. Here is an example of one.
This character is based off of the Crescent Knife from Dragon Magazine issue 275, page 44.
Crescent Knife
Damage 1d3 / Critical x2 / Slashing / Light / Exotic
This weapon resembles a crescent shaped blade affixed to a crossbar handle. It allows its wielder to make two simultaneous attack rolls (using the same modifier) each time he attacks with it. Each attacks is rolled separately.
Character Info: Human
Attributes:
Strength: 18/+4
Dexterity: 16/+3
Constitution: 17/+3
Intelligence: 15/+2
Wisdom: 11/+0
Charisma 12/+1
Level 1:
Ranger 1
28 Skill points
2 Feats: EWP: Crescent Knife, WF: Crescent Knife
Free Ranger Feats: Track, Two Weapon Fighting, Ambidexterity.
Attacks: Str +4/WF +1/BAB +1 = +6
Wielding 2 Crescent Knives:
Primary Hand: +4/+4 Damage: 1d3+2
Secondary Hand: +4/+4 Damage 1d3+1
Level 2:
Ranger 1/Fighter 1
+5 Skill Points
Feat: Improved Initiative
Buy 2 Masterwork Crescent Knives
Attacks: Str +4/WF +1/BAB +2/MW +1= +8
Wielding 2 Crescent Knives:
Primary Hand: +6 +6 / Damage: 1d3+2
Secondary Hand: +6 +6 Damage 1d3+1
Level 3:
Ranger 1/Fighter 2
+5 Skill Points
Feats: Power Attack, Dodge
Add 2 +1 Magic Crescent Knives
Attacks: Str +4/WF +1/BAB +3/Magic +1 = +9
Wielding 2 Crescent Knives:
Primary Hand: +7/+7 Damage: 1d3+3
Secondary Hand: +7/+7 Damage 1d3+2
Level 4:
Ranger 1/Fighter 3
+5 Skill Points
Add +1 to Strength
Attacks: Str +4/WF +1/BAB +4/Magic +1 = +10
Wielding 2 Crescent Knives:
Primary Hand: +8/+8 Damage: 1d3+3
Secondary Hand: +8/+8 Damage 1d3+2
Level 5:
Ranger 1/Fighter 4
+5 Skill Points
Feat: Weapon Specialization Crescent Knife
Attacks: Str +4/WF +1/BAB +5/Magic +1 = +11
Wielding 2 Crescent Knives:
Primary Hand: +9/+9 Damage: 1d3+5
Secondary Hand: +9/+9 Damage 1d3+4
Level 6:
Ranger 1/Fighter 5
+5 Skill Points
Feat: Mobility
Add 2 +2 Magic Weapons, 1 +2, 1 +1 w/Flame
Attacks: Str +4/WF +1/BAB +6/+1/Magic +1(+2) = +12(+13)
Wielding 2 Crescent Knives:
Primary Hand: +10/+10/+5/+5 Damage: 1d3+5+1d6
Secondary Hand: +11/+11 Damage 1d3+5
Level 7:
Ranger 1/Fighter 6
+5 Skill Points
Feat: Spring Attack
Attacks: Str +4/WF +1/BAB +7/+2/Magic +1(+2) = +13(+14)
Wielding 2 Crescent Knives:
Primary Hand: +11/+11/+6/+6 Damage: 1d3+5+1d6
Secondary Hand: +12/+12 Damage 1d3+5
Level 8:
Ranger 1/Fighter 7
+5 Skill Points
Add +1 to Strength
Add a +2 Strength Item (Gauntlets of Ogre Power)
Attacks: Str +7/WF +1/BAB +8/+3/Magic +1(+2) = +17(+18)
Wielding 2 Crescent Knives:
Primary Hand: +15/+15/+10/+10
Damage: 1d3+7+1d6
Secondary Hand: +16/+16 Damage 1d3+5
Level 9:
Ranger 1/Fighter 8
+5 Skill Points
Add 2 +3 magic knives. 1 is +2 w/flame, the other is +2 w/shock
Feat: Improved 2 Weapon Fighting, Expertise
Attacks: Str +7/WF +1/BAB +9/+4/Magic +2 = +19
Wielding 2 Crescent Knives:
Primary Hand: +17/+17/+12/+12
Damage: 1d3+8+1d6
Secondary Hand: +17/+17/+12/+12
Damage 1d3+6+1d6
Level 10:
Ranger 1/Fighter 9
+5 Skill Points
Feat: Blind Fighting
Attacks: Str +7/WF +1/BAB +10/+5/Magic +2 = +20
Wielding 2 Crescent Knives:
Primary Hand: +18/+18/+13/+13
Damage: 1d3+8+1d6
Secondary Hand: +18/+18/+13/+13
Damage 1d3+6+1d6
Level 11:
Ranger 1/Fighter 10
+5 Skill Points
Feat: Combat Reflexes
Attacks: Str +7/WF +1/BAB +11/+6/+1/Magic +2 = +21
Wielding 2 Crescent Knives:
Primary Hand: +19/+19/+14/+14/+9/+9
Damage: 1d3+8+1d6
Secondary Hand: +19/+19/+14/+14
Damage 1d3+5+1d6
And this gets even nastier if you use something like tempest from MotW.