The way I suggested is only really 1 step. Because of the use of the symbolic links, Google Drive and OneDrive will think that they have the folder with all the files in their sync folder, but really it's just a link to the folder you sync with the USB.
I'm doubtful you'll find software that will sync your USB to your computer and upload to Google Drive and OneDrive.
hmmm, it doesn't have to upload, to them just put them in the folder, the onedrive/google drive software do the syncing
thiers other folders in my Gdrive and one -Drive that if i where to do that, it would cause issues, otherwise that would work perfectly
edit: http://www.araxis.com/merge/index.en"paid *Sigh*"