On my /dev/sda, I have Fedora 31 installed.
The layout is:
1 meg spare
300megs /boot/efi
1gig /boot
20gigs /
20gigs /home
16gig /swap

I want to use the next partitions for neon as:
2 efi for neon
2nd /
2nd /boot
2nd /home
2nd swap (not needed but.. have disk space for it)

My question after trying twice to install neon using the 2ne efi partition has met with failure.

Is the initial (Fedora) uefi partition shareable or do I need to use a second physical disk?

Use one ESP (EFI System Partition).
With Fedora & KDE neon installed you should have there at least these folders: /EFI/BOOT, EFI/fedora, /EFI/neon, /EFI/ubuntu (with bunch of *.efi and some other files).