Hopefully I am helping a little and at least pointing out some things that you should find out more about.
On a basic open center system you can only have one circuit or loop. You can have many valves but they all have to be in series. That means you cannnot put a tee at the pump output or any where else either. Think of it this way: The pump is always turning. It delivers, say, a constant 20 gpm of oil. That oil circulates around the loop thru each valve in series. If all the valves are in neutral the oil just passes thru each valve and then dumps back into the tank. It is like an open pipe. The pump does not have to generate any pressure to circulate the oil. If you activate one of the valves, to extend or retract a cylinder, inside the valve the loop gets blocked off and the oil is forced to flow into the cylinder. At the start the oil has no pressure. With the loop blocked off the pump continues its 20 gpm output and the oil pressure quickly builds until there is enough pressure to move the cylinder. As the high pressure oil flows into one end of the moving cylinder low pressure oil is pushed out the other end. This "exhaust" oil returns to the valve and is directed back to the tank thru the low pressure tank return line. The valves down stream of the actiated valve are cutoff from the pump output.
So... if you put a tee at the pump output to make two loops, oil would probably flow around both loops if all the valves were in neutral. The pump would "see" two open pipes in parrallel requiring no pressure to push oil thru. As soon as you activated a valve in one of the loops that loop will, in essance, get blocked off because the flow gets directed to a cylinder which requires pressure to move. The oil will just simply stop flowing in that loop. It will all flow thru the other loop which still looks like an open pipe. The pump will "see" one blocked pipe and one open pipe.
