If your receiver has only one subwoofer output ("Sub Out"), you only need a mono-cinch cable, which is then connected from the receiver to the "Sub In" of the subwoofer. With two subwoofer outputs, you can order a so-called Y-adapter from the Teufel accessories page, which has a cinch plug on one side and two cinch (RCA) sockets on the other. Connect the plug to the subwoofer "Pre Out" on the AV receiver/amplifier and connect the mono cinch cable to the two subwoofers.
If the cabling method is easier for you, alternatively you can alternatively connect one subwoofer to the other. You will need two mono cinch cables - one to connect the subwoofer "Pre Out" on the AV receiver/amplifier to the left line-in input of the first subwoofer, and the other to connect the left line-out output of the first subwoofer to the left line-in input of the second subwoofer.