Two blocks of masses \(m_1\) and \(m_2\) rest on a frictionless horizontal surface and are connected by an ideal spring of spring constant \(k\). A constant horizontal external force of magnitude \(F_0\) is applied to the block of mass \(m_1\), pulling it to the right. At an instant when the spring is stretched by a displacement \(x\) from its unstretched length, what is the magnitude of the acceleration of the center of mass of the two-block system?