Ok, thanks for all your comments guys. Here are revised versions of the two feats along with the reasoning:
Twin Shot
Prerequiste: Base attack bonus +4, Dexterity 15+
Benefit: This feat allows a character to fire two arrows from a bow at the same time. The character makes only one attack roll which applies to both arrows and both arrows are aimed at the same target. If the attack roll is successful then the attacker gets to make to damage rolls for the attack. The use of this feat modify's the attack roll by -4. This feat is a standard action and thus can be used with every attack for the archer however the attack modifier is cumulative so if used as part of the full attack option the modifier for a second attack is -8, a third -12 and so on.
Improved Twin Shot
Prerequiste: Base attack bonus +8, Dexterity 15+, Twin Shot
Benefit: This feat is exactly the same at Twin Shot except that the attacker makes two separate attack rolls and can aim the attacks at different targets if required. Both targets must be with 5-ft of one another.
I've changed Twin Shot so that it can be used with multiple attacks, however the cumulative attack modifier should stop abuse of this, although limiting the feat to a standard action only seems ok.
I've left it as one attack roll for Twin Shot because the two arrows are aimed as one shot, both hit or both miss.
Marshall, DMaple, the point is Rapid Shot requires a full attack option Twin Shot doesn't. IIRC in the film Legolas fires two arrows from his bow in Moria at an orc when they are battling the Cave Troll.