auto mount several SD cards (one after the other) to the same directory

What you're trying to achieve: "ensuring that different disks/data-carriers will be mounted on the same mount point" is exactly what UUID's are intended to prevent.

The solution in your case is assigning a "label" to the file-systems and then using the LABEL= option in your /etc/fstab

Assign the correct labels first with the e2label command :

sudo e2label /dev/disk/by-uuid/c9c87db1-4f03-464d-bfcb-aeec8b3be54d cont_A  
sudo e2label /dev/disk/by-uuid/b444832e-e9c1-4a73-8b3f-94771418e247 cont_A  

or respectively

sudo e2label /dev/disk/by-uuid/7125a2b3-b157-4e65-b618-3b00309b6d21 cont_B

and update your /etc/fstab

LABEL=cont_A  /mnt/cont_A      ext4   defaults,user  0  0
LABEL=cont_B  /mnt/cont_B      ext4   defaults,user  0  0