Pff... There are laser and sniper and turrets on the map and if you can't take a banshee down with those things something must have gone terribly wrong.
And also about your logic of taking single banshee maps out... it fails. Even when I play with one friend on Paradiso or Renegade we can use teamwork to get both banshees easily. We do it like this on Renegade: I grab the banshee and take the attention of the enemy banshee and try to destroy it. Meanwhile my friend is going for the spartan laser with a revenant and usually gets the splazor. If I haven't killed the banshee yet, my friend will laser it. Then I just spawn kill the enemies for three minutes while my friend is sniping or revenant splattering and when the enemy banshee spawns. My friend comes to grab it while I prevent enemies getting it. In one game I got unfrig and my friend go incomceivable thanks to our teamwork. So yeah, more dual banshee maps for me. I love the destruction we make together with my friend in Renegade and Paradiso especially.