Selfhosted

Not in a way you're probably going to like.

You could set up a bare metal hypervisor on the system and set up a VM for your NAS, Windows, and Linux and swap between them as needed, but uh, that's not really an exceedingly pleasant desktop use case, for a number of reasons, one of which is that you really won't have the normal 'sit down, and use the computer' desktop experience.

Alternate option: run the NAS and either the Linux or Windows install in a VM, and keep it booted into, say, the desktop Linux environment with everything else being a virtualized setup.

As others gave said, the solution is a VM but once setup correctly, you won't notice.

If Windows is your primary computer, install HyperV, the built in VM manager for Windows. Then create a Linux VM for your NAS.

Once setup, you won't even notice. HyperV auto saves and reloads the VM whenever you reboot. You don't even need a window open for the VM, it runs in the background until you run the manager to connect to the VM and see it in a window.

If Linux is your primary OS, do the reverse and put Windows in a Linux VM.

Don't hassle with Proxmox, etc. That's for running lots of VM's and toggling between them.

First off you don't need a second drive for Linux. You can dual boot with one drive without issues. If Windows decides to bork it the second drive will do nothing.

Second off, why do you want a NAS OS? Couldn't you just use the second drive for your files? Maybe I'm missing something. I would install Windows and Linux to the first drive and then mount the second drive as extra storage.

Not really, no. I would use either Windows or Linux on the desktop, and run the services and the other OS in VMs.

Personally I use Windows on my desktop, and I have a Linux VM running docker containers. I use that same VM for random Linux tasks I can't do on Windows too.

What you're talking about is a software solution, but the solutions you mention are not standalone software in the way you're thinking.

Honestly, it sounds like you don't want a NAS, you just want shared network storage. If that's the case, make a Fat partition, share it windows, then go configure samba under the Linux side similarly, paying attention to map a user with a matching uid. There will be some wonk happening here and there with file permissions perhaps, but it will work for the most part.

The other options you mentioned are meant to control the entire host, but you may be ready to make that leap yet.

For minimal money, you could also try and get your hands on an older RPi (possibly for free, people just have them laying around), and attach your disks via USB to that, and then you have a basic, but dedicated NAS you can setup the way you like.