The AI won't use the second bridge if you don't have units close enough to it, so blobbing your force next to the single bridge works well enough, and you can later send cavalry to strike the enemy at the rear, as they're attempting a charge. And yes, if you want to, just block the other crossing, as the enemy will usually use only one, and even 1-2 decent spears can hold quite long, so you can reinforce it with other forces.