Yes, after I thought about it for a while I suspected that the second arrangement might sound better. I've never tried the 45 degree arrangement. I assume you actually mean that the inner tweeter would be firing parallel to the back wall, not perpendicular as you say in your second paragraph. Experiment, of course, and see how it sounds, although my guess is that it wouldn't be better. There's no problem with some of the tweeters facing away from the listening position if they can reflect off a wall.
