flow, rate, pipe diameter w/sump pump
The computation of flow through a single pipe driven by two pumps requires some relatively complicated analysis. The flow does not double, because the losses in the pipe increase as the square of the velocity, and the total flow is limited by the pumps needing to overcome the total losses in the system. You can either do the mathematics, which requires a lot of detail (exact pipe diameter, length of pipe, number of bends in the system, roughness of the pipe, elevation difference between the pump intake and the outlet, and the pump output curve) or you can assume you will get about 1.5 times as much flow as from a single pump. Probably close enough.