Chris is correct in that some Sachs have a rear brake like old convential bike, rear coaster brakes that you apply by pushing a bit backwards on the pedals. I have a Sachs Prima and it has such a set up. It only has one barke lever(front) and the other is the clutch lever.
However i looked up a picture of the Columbia Commuter and it looks like it has two levers on the handle bars. I'm not sure if these are both brakes. Try a test on your moped, with the rear wheel off the ground, try pushing back on the pedals like you were going to pedal bacwards and see if it stops, if it does you have rear wheel coaster brakes.
If it doesn't then your two levers are for front and rear brakes.
Generally brake cables have a barrel connector at each end that is perpendicular or at a right angle to the cable, or a barrel connector on one end (the rear brake connector) and a bead or button connector on the lever end. Each of these connectors should lock in to a fitting on the rear wheel and the lever. The rear brake connector should be pretty obvious of where the barrel connector hooks in. it also should have a nut and screw assembly used to tighten the brake cable for adjustment.
The lever end of the cable connects by removing a round sloted pin out of the lever handle. Thread the lever end of the cable into the handle with the end inside the lever. Reinsert the pin to lock the cable in place.
Use the screw mechanism on each of the cable to basically shorten the cable length inside the sheath, thus tightening the brakes. Screw the front lever adjusting mechanism out away from the lever to tighten and screw the rear brake adjustment towards the rear of the moped to tighten.
The simplest solution is to take the moped to your local bicycle shop and they can probably have it done while you watch in about 5 minutes.