I assume you could quickly houserule a feat something like:
"Weapon Combo Focus"
Requirements: Combat Expertise, Two-weapon Fighting, Proficiency with the relevant weapons.
Benefits: Choose a weapon combo, such as rapier and dagger. When wielding both weapons you are considered to have Weapon Focus in both weapons. Does not stack with normal Weapon Focus unless you have at least 8 fighter levels. Does not stack with Greater Weapon Focus.
That could then lead to:
"Weapon Combo Specialization"
Requirements: Weapon Combo Focus, Fighter 4+ levels.
Benefits: Choose a weapon combo, such as rapier and dagger. When wielding both weapons you are considered to have Weapon Specialization in both weapons. Does not stack with normal Weapon Specialization unless you have at least 12 fighter levels. Does not stack with Greater Weapon Specialization.