Append to Library Path

I have this situation where there are certain libraries I have on my local drive, and certain libraries I'm in the process I'm writing that I would like to have in my Dropbox/Box folder, so they are grouped with my other projects and shared across all my machines.

That is, there are certain libraries that are on my local drive, under the ~/Documents/Arduino/libraries folder. But there's another folder ~/Dropbox/libraries folder that I'd also like to include.

As a result, I'd like to have two paths for my libraries directory, but it seems like this is harder than I thought. I tried editing the sketchbook.path variable in preferences.txt, with no luck.

What would be the best way to go about doing this?