You really are not going to get the answer to this question on this forum, you are going to have to do some work. I see two options:
1) You need to combine the system curve with the pump curves. Usually the two pumps are not the same size. You really need to look at a graph to understand. Capacity will not increase at heads above the maximum head of the smaller pump. A second pump will only operate when its discharge head is greater than the discharge head of the pump already running. At a point of equal pressure, the two pump curves join as a combined flow rate, the intersection of this combined flow meets a point from the system curve for total output. You need all four curves.
2) Measure it yourself. Find a nozzle, there must be a drain or fitting somewhere that you can tap into. Start both pumps, fill a bucket, time how long it takes to fill, weigh the water and then calculate into gallons. Look up the weight of water at a specific temperature if you need to be that accurate, but then again you shouldn't use this method if you need that much accuracy.
The depth of the water supplies pressure. Every inch of water supplies an inlet pressure to the pump. Since this is a sump pump, I imagine that the water level changes. (?) So the supply pressure will drop as the water level lowers.
Also, we are assuming that the pumps discharge into a common header. Otherwise they are not in parallel, they would be considered separate systems and you could add the two together as you are trying to do. If they do in fact discharge into a common header, you need to look at the curves or make the measurement.
The pump curves come from the manufacturer of the pumps. The system curve comes from the engineer that designed the system. The system curve is usually done first to choose the pumps. The system curve also depends on a constant pressures, so if the water level is dropping at the pumps, this curve gets hard to follow. In this case, the lower water level will have a system curve, and the higher water level will have a system curve. These two curves will intersect the pump curve at two diffrent points indicating the extreme operating points, and yes, two different flow rates. If you add another pump curve to this, you are going to have a multitude of flow rates at any given time, depending on the water level of your reservoir. What happens when you put two pumps in parallel, of the same size, is that the curve flattens out, thus making the system a little more reliable when system demands change.
I'm sure I have only confused you, but I think this proves Bob's point that he makes so often... get an engineer to help you.
