Speaking from experience, it won't pass emissions because it will not have a complete burn.

The previous post is not entirely true. No auto manufacturer would use two plugs just for the sake of redundancy. Does not make economic sense. Aircraft, sure but cars don't need this kind of redundancy.

I recently had to replace one coil pack on my 2005 CLK 55 AMG with 63,000 miles. Don't drive it very much at all but it developed miss and a new coil solved the problem. Made me think about the purpose of twin-plugs and also whether both plugs in a cylinder fire at exactly the same time or not. Shouldn't be too difficult to optimize their firing if that could make a difference. Anyone know if they fire simultaneously? Thanks.