Prerequisites

Working base install of Radarr installed as a service. (Windows Installer: https://radarr.video)

NSSM (https://nssm.cc/)

Steps:

Open Services and stop the existing Radarr Service (Run services.msc from a CMD window or Run command, double click on Radarr, hit stop and in the drop down set start up to be disabled.)
Unpack the NSSM exe to somewhere you want it to live from now on and make sure you are using the right version for your OS (32 or 64 bit) It seems that any service made with NSSM needs to know where to find NSSM. If you delete or move NSSM after the creation of a service, the service will break.

Open an explorer window to the directory of C:\ProgramData. Find Radarr and I suggest backing up this directory.
Make a new Radarr directory for how ever many instances you need. (My case I made a Radarr-HD and Radarr-4K as I wanted to keep my 4K downloads from overwriting my 1080p)
Open two CMD windows with admin rights and navigate to where you have the NSSM saved.
In the first window issue the command
```
  nssm edit radarr
```

This first NSSM window is meant for a point of reference so you can look back for info for the next two steps.

In the other CMD window, issue the command
```
 nssm install Radarr-HD
```

In path place or browse to

 C:\ProgramData\Radarr\bin\Radarr.Console.exe

Confirm Start up directory is

    C:\ProgramData\Radarr\bin

And here is the key to the multiple instance, place in arguments

    -data=C:\ProgramData\Radarr-HD

Now confirm with the NSSM window from Step 5, that you have matching info in the Details, Log on, and Dependencies Tabs.
Click Install Service
Now start the second one with
```
nssm install Radarr-4K
```
Now repeat steps 7 through 9 with only changing the argument line and matching the other 3 tabs again.

C:\ProgramData\Radarr\bin\Radarr.Console.exe

C:\ProgramData\Radarr\bin

-data=C:\ProgramData\Radarr-4K
Go ahead and start one of the services. (In my case, the 4K one)

Open up the directory associated to the service you choose and find the config.xml

Edit the Config file and change the port numbers. (I just added one more to each port number)
Stop this current service.
Start the other service. If you have more than two or if you just want to change the port repeat 13 and 14.
Restart the first service again.

Conclusion

This should leave you with 2 working instances of Radarr with different databases. You will need to config each one separately. (though if you are adapting this from a working install, you can copy across the other files in C:\ProgramData\Radarr to copy the setup, just leave the BIN directory behind.)

Remember to open up firewall ports for the new ports you chose too.

Thank you to JDIacobbo for giving me a starting point with this (https://github.com/Radarr/Radarr/wiki/Multiple-Instances-on-Windows)

I mostly wrote this for my future self when I need to redo this and I don't really want to have to relearn that one bit of process I am going to forget.

Getting Started

Troubleshooting

Additional Configuration

API