What is the mac model?
How did you split it? Did any of your drives failed?
See this info from macOS terminal for diskutil:
resetFusion
For Fusion Drive machine hardware configurations, reset the disk devices in the machine to a factory-like state (one empty Fusion volume). This command requires the machine to contain exactly one internal solid-state device (SSD) and one internal rotational device (HDD); if so, you are prompted, and if you confirm, both devices are (re)-partitioned with GPT maps and a Core Storage Fusion Drive volume is created. No system soft-ware is installed and no user data is restored. All data on the machine is lost, including any "extra" partitions (e.g. for Boot Camp or other "user" purposes). You generally must be booted from the Internet Recovery System (CMD-OPT-R) or from an externally-connected macOS boot disk (e.g. a USB drive), because you cannot erase a volume with a currently-running macOS. Ownership of the affected disks is required.