[amalcon]

The technology for this does exist, but it's pretty annoying to use in practice. You need to use the "decoy" OS regularly -- preferably most of the time. After all, it's implausible that you haven't used your web browser in six months, etc, and your adversary would notice this.

The problem there is that the "hidden" OS is (by definition) undetectable from within the "decoy" OS. Therefore, you risk accidentally overwriting it. Some encryption software has workarounds for this, but that typically leaves you exposed while it's in use.

Whole-disk encryption is great for protecting credit card numbers, embarrassing information, and trade secrets from someone who should happen to steal your laptop. If you actually have anything so secret that you're worried about being coerced into decrypting it, I don't know how to help you.