If you have, for instance, 500 gph coming from one nozzle, after head pressure and you split it into two lines, you would have 250 gph, minus head loss coming from each one. You are still getting almost the same total gph, but you have distributed it. It would be great if we could add more lines and get the same gph, but it doesn't work that way. You will actually have a little less than you started with because of the additional loss from fittings and any additional head. Keep in mind the diameter of the pipe comes into play here as well, Danner recommends a 1-1/2" pipe to get the full flow from the 9.5 and the flow goes down with smaller piping.
Here is how this figures on a head loss calculator.
Mag9, 1/2" pipe, pumping vertically at 3 feet, figuring no fittings you will get 601.82 gph with 3.3 psi of head pressure.
Mag9 with 1-1/2" pipe, pumping vertically at 3', figuring no fittings, you will get 854.02 gph with 1.36 psi of head pressure.
Add fittings and you increase the head pressure rapidly.