They go the same speed, but the U-2 is at twice the altitude. Due to retinal persistence you will see the part of an aircraft that is longest in the direction

They go the same speed, but the U-2 is at twice the altitude. Due to retinal persistence you will see the part of an aircraft that is longest in the direction of travel (the body) more clearly than the wings. Only military aircraft carry radar, commercial planes depend on TCAS which requires the other object to be transmitting a beacon signal.