John
I think about it as if it’s 2 separate switches, split down the middle. Lets say switch machine 1 changes route A/B and switch machine 2 changes route C/D. With your scenario, you are only changing switch machine 2 and therefore only changing power to rails 3 and 4. Since you are going though route A both times, you are not touching switch machine 1, therefore not changing power on rails 1 and 2.
