how to prepare two SSDs after upgrade (merge to fusion drive or not, terminal commands)
I would recommend to go for the Fusion drive. I am using a Fusion drive with a SSD (128 GB) and a HDD (3 TB) in my iMac for six years now, not a single problem and definitely much faster that a plain HDD. With that experience, I recently bought a new iMac with a 1 TB SSD and added an internal 4 TB Samsung 860 EVO. I combined them to a single Fusion drive (APFS on Mojave) and speed tests show the speed of the NVMExpress bus, which is about four times faster than SATA (with these drives). So far there haven't been any problems.

24 GB is not that much nowadays, and as a separate startup dive, for example, you will run out of space soon. But if you use it as a fast cache for the SDD on SATA, you have 24 GB more space and you can let the OS decide what to put on the faster drive. This will give you some speed advantage.

I would opt for APFS as file system if you use Mojave (10.14) or higher.

Apple has a support document on how to combine two drives to a Fusion drive. According to that document, the simple Terminal command diskutil resetFusion (after booting in recovery mode, of course) should do the trick on Mojave. Unfortunately, this only works if one drive is a HDD. If you want to combine two SSDs, you have to use the following commands (which destroy all the data on your disks!), and it is important to state which drive shall be the main (faster) drive.

Boot into recovery mode (cmd-R) and open Terminal. Look for the names of your drive partitions:

diskutil list


Let's assume the biggest partition on your blade disk is disk0s2, the biggest partition on the SATA SSD is disk1s2:

diskutil apfs createContainer -main disk0s2 -secondary disk1s2


Find out the name of the Fusion container:

diskutil apfs list


Let's assume the name is disk3. So finally create a volume named 'MacSSD' (or whatever) in that container:

diskutil apfs addVolume disk3 APFS MacSSD


Afterwards, you can install macOS again or restore your drive from the backup to the Fusion drive.