I assume you've verified that the SSD has a GPT partition table.

Yes, I did.

Honestly, the easiest way to find out is start over, go to custom partitioning, clock on the blue link to create partitions automatically, and see what it chooses for /boot/efi.

I did this. I chose the SSD-disk, which contains also the Windows boot partition, to install Fedora 28. This is what happened. The installer (custom mode) creates LVM and a /boot partition. Not a /boot/efi partition. Did I boot into UEFI? I think so.

$ ls /sys/firmware/efi config_table esrt fw_vendor runtime-map efivars fw_platform_size runtime systab

$ sudo efibootmgr -v BootCurrent: 0001 Timeout: 2 seconds BootOrder: 0001,0000,0005 Boot0000* Windows Boot Manager HD(1,GPT,2d0b5dac-0a26-4268-8ba5-95dcdae8a821,0x800,0x82000)/File(\EFI\MICROSOFT\BOOT\BOOTMGFW.EFI)WINDOWS.........x...B.C.D.O.B.J.E.C.T.=.{.9.d.e.a.8.6.2.c.-.5.c.d.d.-.4.e.7.0.-.a.c.c.1.-.f.3.2.b.3.4.4.d.4.7.9.5.}.................... Boot0001* Fedora HD(1,GPT,2d0b5dac-0a26-4268-8ba5-95dcdae8a821,0x800,0x82000)/File(\EFI\FEDORA\SHIMX64.EFI) Boot0005* Fedora HD(1,GPT,2d0b5dac-0a26-4268-8ba5-95dcdae8a821,0x800,0x82000)/File(\EFI\FEDORA\SHIM.EFI)..BO

I found it and verified that it works. On the hard drive selection screen, in the bottom left corner, there's a boot selection link. Click that and you will get a dialog box that will let you select the other hard drive as the boot drive. Then it won't complain about the EFI partition.

So If I reinstall the way you suggest, and choose the HDD-disk as the boot drive, can I still go on with " custom" installation an put / on the SSD?
And shall I still have the advantage of a fast boot?