Ok, Draw yourself some plumbing
Two pumps, One sucks out of a tank, the other pumps back in
The lines from these pipes flow to devices that are powered by the flow of water (Turbines) and then to a common "T" connection.. The 3rd leg of the T connects back to the tank
For ease let's have the left hand pump the pressure pump Sucking water from the tank into the pipes, and the right and the suction pump, returning water to the tank.
Fire up both pumps..No water flows due to the turbines being turned off.
now we open one turbine, Let's make it the left one.. Water flows out and back on the "T" Line
Of course there is some pressure loss both ways due to friction with the pipe.
Now let's close that turbine off and open the other one.. again water flows.. This time FROM the tank, via the "T" line,through the turbine and back to the tank.
Now... Lets fire up BOTH turbines.
Water flows out to the left turbine the to and through the T, to the other turbine, then back to the tank.. NO WATER FLOWS IN THE "T" line
That which flows out on one side returns on the other
Oh, I mentioned pressure loss due to friction.. Well it is still there but now it is effectively halved (Since half of it is on both legs.
And that not only explains what is happening... but why as well.