The DF2-Card is not strictly a floppy drive controller, what it does do is allow the installation of a third internal floppy drive in the A2000. Most Amiga's support up to four floppy drives, but have a limit on the number of drives which can be fitted internally. Usually 1 or 2 depending on the model. The DF2-Card supports both 3.5" and 5.25" drives. It doesn't work by simply re-routing the external floppy drive port using some form of cabling, but rather it has an "electronic bootsector". When the Amiga reads this bootsector, it reboots the machine and makes the drive appear as DF0:, as Amiga's running older versions of Kickstart will not boot from floppy drives other than DF0:

Thanks to Iggy Drougge