Well, I can see why a person would find this useful, however, I do not think that it is at all possible.
Most online storage sites are a bit slow, especially if you have a slow connection. Copying and pasteing data takes time.
Most faster services require that you pay a price, search for "Online Backup storage" on Google.com
Can you raid two together?
No. FTP/HTTP Protocols are much too different for this type of action. It would be best to use the first until its full, and move to the second.
Being that its FTP/HTTP, it would move way too slowly if you tried to raid the drives, two together would not make it faster. Attempting to make it look like a physical disk will not make it faster.
A virtual hard drive, or VHD file, is used in emulating a different operating system on another computer.
Executing a file on FTP/HTTP would do the following:
Open its properties.
Yup, FTP is that useless...
It can open some files, so MP3, image, maybe movie, and documents would open, but many files, especially executables, would definitely not execute properly.
As for sharing:
If you had the ability to add FTP accounts to your server space, then yes, you could set permissions for your friends/family to read data, but to not edit or create data. You could also give your best friend full access to one single folder, or allow anyone to access your public folder (if it was set up) and you could use anonymous access for anyone to read data.