Truecrypt had this exact function - one password would decrypt your drive sort of on one end, and start the OS there, another password would decrypt the drive on the other end, and start the OS installed there - so you always had perfectly plausible deniability, since the drive taken as a whole looked like a completely normal encrypted drive(in fact you could accidentally destroy the hidden partition by overwriting "empty" area while booted into the non-secret OS). Always thought that was super cool.