Merge original Fusion Drive SSD and 3rd party internal SSD

Hi all,

One year ago my Fusion Drive partially broke. The Fusion Drive-HDD broke, the 128Gb SSD from the Fusion Drive was still working fine. I ordered an SSD which would be placed inside my iMac by a guy that knew how to do it. Afterwards he did some lines inside Terminal to merge the Fusion Drive-SSD and the new internal SSD as one "Macintosh HD" with a total of 2,128 Tb.

One week ago I wanted to do a clean install and I accidentally undid this merge within the DiskUtility inside the Recovery-mode (Cmd+R) of Catalina; I somehow unmerged it. Can you guys tell me to merge it again as one disk where I can install macOS on? Now I installed Catalina on the "new" 2Tb SSD, but I want to merge the SSD with the original Fusion Drive-SSD that's still inside. Unfortunately I didn't watch the guy do it in detail so I don't know how he did it exactly. Is there a way to do this inside the UI of DiskUtility or only via terminal using diskutil commands.