No way this would not be a good idea; for two reasons. 1. Way less war loot! 2.Some people fail their 1st attack and/or their first choice gets taken and they are forced to attack a less then feasible base, so the second one really helps for them.
Most people I talk about this with feel that 2 attacks in a 50 vs 50 clan war is not enough. If the bottom half don't have a target maybe let them attack first or get your own clan to attack someone closer to their ranks.
thats a bad idea. If the top half of your clan are all attacking the bottom half of the enemy clan, find a different clan to join, cause those guys are horrible people.