Cannot use Gotek as 2nd drive to copy from real disks

[gf65]

Hi,
I have a Gotek with the rotary and lcd screen (chipset STM32F105). I updated it with FlashFloppy 4.7a.
It's connected as an external drive on my CPC6128 Disk Drive 2.
When I don't connect the Gotek, I can use my floppy disks from the built-in drive. When I connect the Gotek, I can load disks however it is seen by the Amstrad as both the A and B drive.

I'd like to dump/copy my disks and I can't find a way to have both the built-in drive and the gotek to work. Any idea ?

Could there be Gotek models that don't allow switching between A & B ? Is there something to do on the cable ?

UPDATE: tested on a friend's CPC 6128 and it works. What could my CPC have different that prevents from using both the internal drive and the Gotek ?

Extract of FF.CFG:
interface = shugart
host = unspecified
side-select-glitch-filter = 0