Good day, a fusion drive has two disks -- an SSD and a standard hard drive. If you separate them, you can not access the drive. At this time, you're suggested to restore the drive to the original and then connect the one iMac to enable Target Disk Mode to grab data stored on the iMac.
If you fail to do so, this article comes here to help: How to recover data from crashed/dead MacBook Pro/Air?