Although there is no limit to the number of Dropbox accounts you can create, you can run only one instance of their desktop client on your computer. If you have more than one Dropbox account and need to setup all of them on your computer at the same time, follow this tutorial.
For Windows 7 and Vista
To run more than one Dropbox instance, you will require more than one user account in Windows. If you don’t have one, create a new user account in Windows. The alternate user account does not need to be an administrator account, just a normal user account will do as long as it allows installation of programs. This user account must also be password protected.
Install your first Dropbox account with your primary windows user as usual. Now log off your account and login again through the new user account. Once logged in, download and re-install DropBox as the new user. You may have to enter the admin password here if your new user account doesn’t have administrator status.
Configure your second Dropbox account under the new user, but make sure that both Dropbox account uses different folders.
Log off your new user, and back on as your primary user. Now we need to create a custom shortcut to the alternate DropBox instance. On Windows 7 and Windows Vista, the DropBox executable is stored under the local users AppData directory.
Once you locate the executable, right click it and click Send To > Desktop (create shortcut).
Right click the new shortcut on your desktop and click "Properties." In the "Target" text field append the following to the beginning of the command:
Replace USERNAME_SECOND with the actual username of the account. After editing, the “Target” field should look like this:
Click OK to save the changes to the shortcut and then double click it to run. You will be asked to enter the password for your second user account.
Enter it to login to your second Dropbox account. At this point you will see your second DropBox icon appear in your system tray.
If you have another Dropbox account, create another new user and repeat the entire procedure.
For Windows XP
Windows XP users can use Dropboxen, the official Dropbox addon that will let you run more than one accounts at the same time. Dropboxen doesn’t work on Windows 7 and Vista.
For Mac OSX and Linux
Tutorials to run multiple instances of Dropbox on Mac OSX and Ubuntu are on Dropbox Wiki.