Win 7 Pro or Ult can do spanned volumes. Basically JBOD. There's no restriction that the volumes be the same size, so you could glom the 256 and 512 GB SSDs into what looked like a single 768 GB SSD.
http://technet.microsoft.com/en-us/library/cc772180.aspx
Unfortunately, I don't think you can boot from a spanned volume. And since it's not fault tolerant you basically double the chances of losing the contents of the SSDs. You don't get the speed benefit of RAID-0 either since the data isn't striped (half written to one drive, half written to the other). The OS just treats both drives as one contiguous disk. Your files will be written to one drive or the other, with few if any files straddling the two drives.
What exactly is it that you're trying to do? If all you want is to be able to access the contents of the second SSD via the same drive letter as the first SSD, you can create links/junctions to mount folders on the second SSD as folders on the first SSD. This is a little less flexible than a spanned volume (you have 256 and 512 GB capacity limits, instead of an aggregate 768 GB capacity limit). But it's safer and you can control which files go on which drive.
http://ipggi.wordpress.com/2009/09/07/windows-file-junctions-symbolic-links-and-hard-links/
I use this to trick Steam (which insists on putting all my games in the same directory tree) into installing some games on my SSD, other games on my HDD. And it's simple enough to rearrange because it's just a bunch of links. If I finish a game that's on my SSD, I can copy it to my HDD, and create a junction from its folder on the HDD to the original folder on the SSD. Likewise if I want to move a game from the HDD to SSD, I simply delete its junction on the SSD and copy the game's folder from the HDD to the SSD. The OS thinks all the games were installed in C:/Games/Steam/* regardless of whether the files actually sit on the SSD or HDD.
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