The flow dynamics and heat transfer of natural convection in a horizontal, initially quiescent, uniform temperature enclosure layer with perfectly conducting side walls is numerically simulated. Symmetric heating and cooling are simultaneously applied to the lower and upper walls, respectively. Rolls form at once near each of the vertical walls. These rolls propagate horizontally in two rows, one above the other, sustained by the local temperature gradient created initially by the side walls. This flow pattern, which occurs in a wide range of Ra, reverts to a single row as alternate rolls merge. At low Rayleigh numbers convection becomes manifest after the final roll pattern has formed, whereas at higher Ra convection sets in during the propagation phase of the initial, double train of rolls. Two and three dimensional solutions are given. Results are compared with previous work and with three dimensional solutions. Heat transfer results for the final regime are given.