Nested encryption is usable as an access-control, but doesn't help against all the other problems you get with sharing identities/systems which is something you may want to consider as well.
Handing an unlocked laptop over to someone else is pretty much always a bad idea, if only for attribution. Don’t share accounts, including computer accounts. Pretty much all modern systems allow for easy multi-user computing with fast user switching. If you both need to work on the same file, Apple, Microsoft and Google support this using iWork, Office and Docs.
Computers and user accounts are personal items. Treat them like your underwear: you should be the only user.
To elaborate on the 'handing over an unlocked system' part, this basically means that you are no longer in control of what happens from that point onward. Even without malice in mind, this is not something you should accept. Because the system is 'yours' and your account is 'you', this means that everything that happens on that system is attributed to you. At the same time, many external systems might still assume that if it is your system accessing or doing something, it must mean it is 'you' doing the thing or accessing the stuff. It cannot differentiate who is at the controls, only what requests are made and served.
You hand over your unlocked system to someone else and need to do something somewhere else where you no longer have the machine and its user in sight. At that point, the (non-malicious) user might be doing the intended thing, like looking up some data or filling in some forms. A popup might appear, or a site might be visited which after interaction or sometimes even without interaction persistent malware is introduced to the system. The user might not know this, or might not care, or forget about it or even assume this is intended as your system might be setup in a way that is different from what this user is used to. You return and after the user is finished, you get to work on your machine. The persistent malware now tracks all activity and some NDA'ed information gets leaked. This information just happens to be watermarked with your identity, and turns up somewhere. Now you're on the hook for leaking information that should not be publicised and they can prove it is yours because of the watermarking. Now it is up to you to prove that it was stolen/extracted/leaked and not something you intended to have happen.
While this often can be resolved, it will still take a lot of time, resources and attention which could have been spent on other things of your choosing.
Instead of having anything like this happen, you could also just setup the system in a way that prevents this from happening in the first place, but still retain the possibility to work together on the same data or applications. This can be done in a number of ways, for example:
- Work on separate accounts, share data as needed
- Work on systems that are intended for collaboration
- Use the OS-provided guest account and share data using the guest-accessible storage methods (i.e. the read-only Public and write-only Drop Box directories on macOS)
Many of the security-related issues translate into possible wastes of resources/time in case something does go wrong, and not immediately loss of resources/life/access. Preventing something from happening by using a suitable method of collaboration is the key in most cases. While this answer is rather large and possibly contains duplicate information from other Q&A posts, I hope this describes enough reasons to look for better ways to achieve your goal.