Use the two wrenches to loosen the nut that holds the brake cable to the braking mechanism. Grip the nut with one wrench, then grip the bolt (on the other side) with the second wrench.

You might want to enlist a friend to hold on to the end of the cable for you so it doesn't fall off when you loosen the nut that holds it down.

In this picture we use pliers and an adjustable crescent wrench in place of the 9mm wrenches that would be ideal for that bolt. However, anything that can grip and turn the bolt will work just fine.