Why don't humans need two hearts to pump blood through our lungs?

Because the heart consists of four chambers. each chamber pumps oxygenated and deoxygenated blood throughout the body. two hearts could be very well dangerous due to the excess amount of oxygen in the bloodstream.

I agree with the first answer, but to expand : the left ventricle pumps oxygen rich blood to the body (through aorta) and the right ventricle pumps the deoxygenated blood to the lungs.

Anyways the reason two hearts aren’t necessary is because you have more than one chamber in the heart. The left ventricle is bigger with thicker walls because it pumps blood throughout your entire body. The right is smaller because it just pumps to the lungs and back.