Yes, when sharing the project over Dropbox on a Mac you wouldn’t be able to see the full photos on both machines, unless you were using the same username as Dropbox is typically setup at /Users/<your name>/Dropbox. It might be possible set up Dropbox at a neutral location, e.g., if you could configure your Dropbox to be at /usr/local/dropbox on both your machines then you should be able to share the project that way. If you do this now, you will also have to update all the paths currently saved in your project (the best way to do this is currently a one liner in SQLite).
Sharing via USB should also work if you can ensure the path to the device is the same on both Macs by giving the device a unique name. Again, you will have to update the paths in the project.
Please note that using the same project over Dropbox should be fine, but you have to be careful if you’re editing the project at the same time (or if you edit it while offline, and the other person makes changes before your computer goes online again) because it will be easy to create sync-conflicts on Dropbox. We’ll be looking to find a solution for this setup, but in the meantime please be careful and make backups of the project file.