When two pumps lead into the same system by connecting their discharge nozzles, they are now essentially pumping to the same system and the flows from each one will be determined by the back pressure from that system into the pumps. Therefore there is practically no difference in the head at which each pump will be operating, regardless of the size of the pumps. You should be careful however, to consider the shape of the pump performance curve and determine the flow rates at which each pump will be operating as this will determine the level of efficiency and reliability you will achieve, assuming of course, that they pump at all.