Hi Michael, merry xmas to you too 
The drive-manager recognize only one of the two
Strange, does it not show up in the menu after refreshing the drives? Also, what filesystem is on the drive?
but I’m not sure if the Drive-manager overrides my fstab on boot time?
Only when running dietpi-drive_manager will it modify /etc/fstab. So you can edit and add drives to /etc/fstab manually, disable the DietPi entries and dont use dietpi-drive_manager again, should be fine.