Skeletal muscles work in pairs because they can only contract and cannot actively lengthen. This is known as the muscle's contractile nature. When one muscle in a pair (the agonist) contracts to create movement, the opposing muscle (the antagonist) must relax to allow that movement to occur. This complementary action is necessary for smooth and controlled movement around joints.
So, while it's true that muscle cells can relax and contract, the primary reason for skeletal muscles working in pairs is due to their contractile ability and the need for one muscle to counteract the movement produced by the other.