Installing Bootcamp on second internal iMac Drive?

• Create a Time Machine backup of your macOS installation - Back up your Mac with Time Machine - Apple Support.
• Boot into Internet Recovery - About macOS Recovery on Intel-based Mac computers - Apple Support
• (Re)Build the Fusion drive - How to fix a split Fusion Drive - Apple Support. Be aware that you have two SSDs, and the 2TB SSD should be the designated Secondary disk. You may need to use Utilities -> Terminal in Internet Recovery
• Restore your TM backup - Restore your Mac from a backup - Apple Support
• Install Windows - Boot Camp Assistant User Guide for Mac - Apple Support

If you run into any issues, please post back.

You will need to create a Fusion drive with the NVMe as the 'main' and the SSD as 'secondary'. This will allow BC Assistant to use the SSD for Bootcamp installation.

See the reference to the Bootcamp partition in the following entry for diskutil.

diskutil apfs createContainer 
Usage:  diskutil apfs createContainer <disk> [<disk>]
        diskutil apfs createContainer -main <disk> [-secondary <disk>]
        where <disk> = MountPoint|DiskIdentifier|DeviceNode
Create an empty APFS Container. You can then add APFS Volumes with the
diskutil apfs addVolume verb. If you specify two disks, then a "Fusion"
Container is created, with the performance usages assigned automatically
unless you use the -main and -secondary options, in which case, the secondary
disk is assumed to be on "slower" hardware. The secondary disk is often used
to store associated "auxiliary" data, such as a Boot Camp Assistant partition.
Ownership of any affected disks is required.
Example:  diskutil apfs createContainer disk0s2