The reason you couldn't have 2 J shaped holders, is because the screws that hold the J shape on would be behind the marquee, so you would not be able to get the marquee into them once they were both mounted.
If you have a J shape on the bottom, then you get the cleaner look of the J, as you would not have any of the metal visible on what is the speaker mount on most cabs, but then you can have the L on top, so you can actually access the screws to remove it.