From the page you referenced:
"I built Moppy using an Arduino UNO, though it should work just fine on most Arduinos. The pins are connected in pairs to floppy drives as follows: Even pins (2,4,6...) are connected to each drive's STEP pin, the matching odd pins (3,5,7...) are connected to the each drive's DIRECTION control pin. So the first floppy would be connected to pin 2 & 3, the second floppy to 4 & 5, and so on."
Looks like you need three wires to each drive: Step, Direction, and Ground.
"Some pinout information can be found here: Floppy Diskdrive pinout and wiring @ old.pinouts.ru
They don't say it but I think that ALL the odd-numbered pins on the floppy connector are Ground. Pick any one.