Question: Hidden Installation of Windows

I am working on a solution for a friend concerning his laptop. He proposed an idea to me and I would like to see it through, but I'm not quite sure how I am going to execute it.

The idea is this: To modify his boot up sequence so that when he just turns on his laptop it boots into an instance of Windows 10 that looks and feels normal, has a bit of internet browsing history, a few files here and there, but in reality this isn't the instance he uses. He actually uses a hidden installation of windows 10, encrypted and secure of accidental discovery by pressing a key sequence at boot-up or something of that nature. That's the part I am fuzzy on. I know for Linux you can actually install the boot-loader on a separate drive and you boot from that drive to get to your OS. That's actually how I boot my machine, but I don't know if this is possible with windows. Actually I'm not even positive you can have two installations of the same version of windows without them wanting to eat each other.

I suggested the flash-drive option to him and he wasn't too thrilled about it, but if it's the only way, I think I could convince him to get one of those flash-drives that are the size of wireless mouse receivers. That way he can set it to automatically boot from USB, but as soon as he takes it out, it returns to decoy mode. The ideal solution would be to not have the flash-drive at all and just hold a key down or something.

So the full list of requirements would be as follows:

• Hidden - Undiscoverable by someone who boots the laptop
• Encrypted - If someone does find it, they can't access the data
• Same OS as the decoy
• Preferably not have a flash-drive to maintain

Any help on this topic would be appreciated. I would like to know, first and foremost, if this is possible and what tools you might use to accomplish this. Thanks