I would suggest to do the following. (1) A Time Machine backup (or 2), Maybe one to a network storage and one to a local attached usb/firewire disk. (2) Then replace the 2 disks. (3) Do a fresh install of Catalina, you will get an option to choose one of the new disks. (4) During install use to option to Recover from a Time Machine backup and use the one on the local attached disk. Some more detail of the fusion drive. If you go into terminal and type: % diskutil list You will see something like below. it shows your current ssd and hhd, /dev/disk0 and /dev/disk1 in the list below. After that you see another disk which is: /dev/disk2 (synthesized) which is the fusion drive. That is where your information is stored. When you do the fresh install, the install proces will create the required disk layout with the recovery partition etc. Diskutil list output: /dev/disk0 (internal, physical): #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme ...
Yes you can split the fusion drive. You can even do it before you replace the drivees. See this article: https://www.lifewire.com/split-fusion-drive-apart-2260166.
