Setting the temperatures would probably be easier than balancing the flows if they are connected in parallel.
The first heater (the one with the cold water feed) is almost always going to get the most work because it will immediately try to heat the water. The second heater will maintain its temperature and will raise the water temperature when the high demand exceeds the capacity of the first heater.
The two heaters will have double the recovery rate when you have high demand. That is the advantage of having two heaters versus one heater and a storage tank. If you just want a storage tank you could use an electric heater set at a temperature a little lower than the first stage gas heater. I think you will get less heat loss from an electric heater, but you will lose the advantage of the higher recovery rate and lower energy cost of the gas heater.
If you are going to have the capacity you are buying with the second heater, you will want them both at the operating temperature that you need to supply your demand.