You can add two SSD’s to create a fusion drive. However there are important caveats.
Apple list a specific set of terminal commands to create a fusion drive set. These do not work when using two SSD’s, the older set of steps easily found online at MacWord etc do work and will create a fusion drive from two SSDs.
However, Apple specifically block this configuration and the installers for both Mohave and Catalina will refuse to instal if you have two SSDs as they will see the array as a simple RAID setup and macOS no longer installs on RAID drives. You can instal High Sierra, this seems happy enough to work but any future upgrades will need to be done by cloning back and forth using 3rd party tools which is a pain.
The main advantage from using two SSDs is not going to be a lot of speed as the FD should be running from it’s SSD component most of the time (less so on post 2017 iMacs where the SSD was dropped from 128GB to 32GB (really???), however on 21in iMacs even fusion drives suffer from the awful spec drives Apple used in these so you will gain some speed but more importantly reliability.
So swings and roundabouts.