zzefer,
The short answer is they would be used separately. Basically you would set up your model and use the data from those hydrants to calibrate your model. First make sure that both hydrants give you the same static reading without flowing anything in the system. Then further develop your model by making one hydrant flow at a time. In other words, make one flow and make necessary adjustments to you model (pipe roughness/length adjustments/minor losses) until you reach the residual pressure from your reading. Then do the same to the other. One big heads up, assuming that there were no issues on your hydrant flow tests, or someone taking a bad reading and that your model is set correctly - simply doing the calibration with one hydrant should get you very close without having to do much adjustments to your system when checking the second.
Once everything is calibrated you would only use one to deliver water to your system when modeling say the new system that you are tying into. Hope this helps.
