This can be done quite easily by downloading the data from the first cloud onto your computer, then uploading it onto the second cloud. However, this is slow and tedious, especially if you want to move large amounts of data. Luckily, there are a few online cloud services which enable this transfer to take place without having to download and upload all that data. Some of the best services are Mover (mover.io) and MultCloud (multcloud.com). Using a tablet? Look at apps like AirFile or CloudHQ. These services will require you to sign in with the accounts of both cloud services, enabling you to transfer data between the two accounts.