It hits only one additional enemy within 70 range of the *target*.
No cone-shape here, it's a straight circle around the target. I figured it made more sense to make it like that instead of taking the berserker himself as a reference (this means that when enemies are not standing close to each other, but both close to you, you will only hit one target).
In fact, the actual range is more like 100 instead of 70. I figured 70 was a little bit too small, so I increased the range somewhere in the design process. I could have changed to tooltips, but was too lazy to do it. It's not like anyone could measure it anyway...
One another side note, the enemy hit by the cleave effect is not random. It always hits the closest enemy to the target. The additional target hit by the talent will hit the second closest enemy.