I'm not sure that would work. Someone else can chime in here that maybe has done the same thing and give you their recommendation. The problem as I remember it from other posts is that there is no way to connect both the SSD and blade SSD in the same configuration as was with the fusion hard drive and the 32GB blade SSD.
I haven't done it myself.
In the 27", you can only install a SATA HDD/SSD if there was a HDD installed at the factory (either HDD or Fusion Drive). You can always install a blade SSD whether or not it was installed at the factory.
With the 21.5" you can only install a SATA HDD/SSD if there was a HDD installed at the factory (either HDD or Fusion Drive) and you can only install a blade SSD if there was a SSD installed at the factory (either SSD or Fusion Drive).
So any iMac that had a Fusion Drive installed at the factory or in the case of the 27" a Fusion Drive or just a HDD, can have both a SATA SSD and blade SSD installed.
Apple manufactured logic boards in such a way that for e.g. a 21.5" model with just a HDD, a blade SSD couldn't be installed. There isn't the connector for it.
There may be some other limitations as to whether a drive is compatible, for instance OWC's Aura Pro X2 isn't compatible with late 2015 iMacs. Perhaps that might be related to boot rom updates for the 2015 iMacs failing to be installed for a number of those Macs.
Before installing 3rd party drives it would be recommended to update the bootrom to the latest version. The latest version can be found in security updates for the previous 2 versions of Mac OS or software updates for the current version, so Security Updates for Mojave and Catalina and updates to Big Sur. For a number of Macs there have been bootrom updates improving compatibility with 3rd party NVMe SSDs fixing hibernation issues.