Unfortunately, again, I didn't have time to complete my thoughts when I posted my last message.
To elaborate a bit, my thought was that you want a system that essentially emulates a conventional RAID 1 system except that one of the drives is "in the cloud" (ie, remote, but a drive letter can be assigned as if it was a local, physical drive).
There are several software products that can do this. The first that comes to mind is:
ViceVersa Pro .
Although I use this product, I only used it to automatically (and incrementally) backup one local drive to another local drive, not a remote drive, but they state:
"ViceVersa works between computers, e.g. Laptop, Desktop, Workstation, Server, over network LAN, WAN, VPN, USB, and with any type of storage media including external Hard Disk, Zip disk, USB flash drive, CD-RW, DVD, NAS (Network Attached Storage)."
"Auto run on folder content change, real-time. Real two-way file synchronization: if a change occurs in either location, it is reflected on the other side."
so, I think it should do exactly what you want.
HTH,
Tom M
PS - If this doesn't work for you, I'm pretty sure there are several other similar products on the market.