I once had a professor that would frequently say "as is obvious to the casual observer..."! There are many things about wiring that are not always obvious. You have correctly phased the two Z4000's supplying track power. The downside of using a common return is the current (amps) in the return is the sum of all the current in the four supply channels. So if two handles are supplying 4A and two are supplying 2A during operation the common return is carrying 12A, greatly increasing the voltage drop unless a much larger return conductor is used.
If there are transformers supplying loads like lighting , these do not have to be in phase. One can use two separate transformers for these kind of loads and have them 180 degrees out of phase. Then, if each transformer supplies 4A the sum of the currents in a common return is Zero. This works for loads that do not need to be in phase, not for track power.
