Disk Utility Partitions Wrong Part of Fusion Drive
To create a proper "Fusion Drive" in your case, you first have to create a normal CoreStorage Logical Volume Group with one physical volume and one logical volume. Then you have to install macOS. In a last step you have to add the second physical volume to the CoreStorage Logical Volume Group and expand the existing logical volume.


backup your main volume
boot to Internet Recovery Mode (or a macOS installer thumb drive)
get all necessary listings:

diskutil list
diskutil cs list

destroy the LVG:

diskutil cs delete lvgUUID #lvgUUID: UUID of the Logical Volume Group

completely erase both disks (each to one volume with the names e.g. "data1" and "data2" but not "Macintosh HD")
get the device identifiers again: diskutil list
create a new LVG and LV on the larger disk (here I assume the 500 GB SSD has the disk identifier disk0):

diskutil cs create Fusion disk0
diskutil cs list
diskutil cs createVolume lvgUUID jhfs+ "Macintosh HD" 100%

Install macOS to "Macintosh HD"
After configuring macOS open Terminal and enter

diskutil list
diskutil cs list

add the 24 GB disk (here I assume the disk identifier is disk1) to the LVG:

diskutil cs addDisk lvgUUID disk1

Expand the main system volume:

diskutil cs resizeVolume lvUUID size #with size e.g. 526800m


If you get an error "Error: -69720: There is not enough free space in the Core Storage Logical Volume Group for this operation" choose a slightly smaller size like 526400m or 526000m