the Ubuntu One client is designed to only support a one account per OS user and the way to handles the configuration files, there's no clean way around this.
If you want to switch to a new account without going back to the other account (on Ubuntu):
- Make sure all your files are in sync.
- Backup the files syncing with Ubuntu One (Important in case you mess up any of the following steps)
- Stop ubuntuone u1sdtool -q
- Remove the Ubuntu One metadata: rm -rf ~/.local/share/ubuntuone
- Remove the openid token in the keyring: run seahorse and delete the key named "Ubuntu
- Restart Ubuntu One: u1sdtool -c
- When prompted to login, login with the other account.
Ubuntu One will begin syncing the files to the other account. It may be a good idea to make sure the other account doesn't have the same files. Otherwise you could end up either with a lot of conflicts or in some cases, delete files (put them in the trash)