No, this is not possible and we have already decided against implementing this feature. When we develop a feature it has to work under all backup engine configurations. The trickiest configuration is the "Upload each part immediately". It requires having code in too many places in the backup engine to support it. Having two post-processing engines would only make this code even harder to maintain and more prone to difficult to debug bugs. Moreover, failure to upload to one engine could be treated as a backup failure - or not. If it's not, however, we have a problem: how would the user even know that the upload failed to X but worked in Y? What if X is your client's copy?
There are dozens of more, more subtle and more serious issues surrounding such a feature. So, as I said, we have decided against it.
You can do something simple. Create a Google Drive or Dropbox account (or S3 bucket, or...) that both you and your client have access to. I'm pretty sure you want to protect your clients from themselves. That's simple too. Give them read-only access to the backup upload location ;)
Nicholas K. Dionysopoulos
Lead Developer and Director