Most implementations so far have been kind of shaky (e.g. relying on having a bunch of "unpartitioned" space on the drive, which would seem weird), but it's technically feasible to have a single encrypted volume which decrypts to two results based on the key. For plausible deniability, one would have to make sure the clean volume had recent browsing history, et cetera. Maybe this is a real-world use for ad-browsing bots.