Each situation is different, and the rooster's personalities would affect my decision of keeping two or not. I had two roosters with 12 hens and they didn't fight at all, but I don't think they did better than one would, as far as predators go. 
 
Now I have two different roosters with 22 hens and somehow they work perfectly together - they're huge - one is a Dark Brahma, the other one is his son, with a Buff Orpington. 
When they spot a hawk, they give the alarm and while the hens hide, they stay in the middle of the field, screaming their heads off at the hawk, until it leaves the area. 
 
They are extremely aware of their surroundings. Just two examples:
 
One day my husband heard the older one growling (his son wasn't even born at the time), and went to check out. All the hens were hiding under the coop, and the rooster was staring at a red-tail hawk, who had landed about 15 ft away. My rooster was just staring at the hawk, hackle feathers up, and making this low, angry growling until the hawk flew away, as my husband approached. 
 
The younger rooster actually attacked a fox before I even saw it - he ran at the fox literally growling, hackle feathers up and flapping his wings. The fox was so surprised it ran away instead of attacking. 
 
But I chose these two out of six roosters, based on their behavior. So it really depends on the individual rooster/roosters.