My first toon was a Gunslinger so I ended up using more than two bars in normal use.(my slinger seems to use more items than any of my other toons)
I have my bars arranged all horizontally. The first two bars are just as they are by default, as I got used to that, and the vertical (side) bars have been rotated to be horizontal with below my first two bars. So kinda like ::
My main weapons/tools are on the default bars, with the less used weapons (healing, etc) beneath them on the middle of the bottom bars, and finally the really rare stuff at the ends (like Heroic Moment, Sprint, and Fleet Pass).