I did this earlier today on an iMac. Boot to a Mojave USB boot installer. Then Utilities —> Terminal. The commands are along the lines of:
diskutil apfs list
Then make note of the “virtual disk” that is the “Fusion Drive”. This is made up the PCI Flash and the other spinning drive. You can see how the text output of the drive structure is hierarchical.
Let’s say that virtual container disk is identified as “disk0”
diskutil apfs deletecontainer disk0
This command will split your fusion drive, and leave you with two new drives to format via Disk Utility (which you can run in the same USB Booted environment you’re already in).
Format them (the SSD and the other drive) as whatever you want.
I recently did this prior to replacing the slow spinning disk that comprised the 1TB of my Fusion Drive. Replaced that with a 500GB SSD which is SATA3. Installed Mojave onto the 128GB PCI “Blade” Flash and formatted the other SSD.
I then store User home folder on the larger SSD, and macOS and other apps run off the faster PCI-based SSD.