FastFileSystem is not ROM resident inside the KS1.2 or KS1.3 Only with KS2.04 and above was the FastFileSystem made into the Kickstart ROM.
Your a500 with KS2.04 will recognise OFS, FFS and many other file formats, where as the A2000 with KS1.3 can only boot OFS (OldFileSystem or OriginalFileSystem) disks.
The file formats are different, the total storage of the disks is different. The code in the boot sector is different. You cannot "boot" from a disk the ROM cannot recognise.
KS1.3 Harddrives recognise FFS only when the FastFileSystem file itself is stored in the Rigid Disk Block.
If you need files from one machine to the other, format a floppy on the A2000 (OFS) and copy files to that floppy from the A500 (FFS)