Imagine that one has a loop of inelastic pipe which allows water to flow without friction except at one point, where flow is proportional to pressure difference.
On this loop one has two pumping engines; one will ensure that five gallons per minute of water will flow through it, and the other will ensure that ten gallon per minute will flow through it. Each engine will add or remove as much energy as required to make the proper amount of water flow.
If only the 10g/min pumping engine were present, the pressure difference between its upstream and downstream sides would be whatever was necessary to push five gallons of water per minute through the constriction. The tighter the constriction, the greater the required pressure difference, and thus the more energy the pumping engine would have to add.
If a 5g/min engine were added in parallel, pumping in the same direction as the 10g/min engine, it would increase by 50% the amount of water that needed to flow through the constriction, and thus increase by 50% the amount of energy needed to make that happen. The total amount of energy dissipated by the constriction would be increased to 2.25 times the earlier amount; the 10g/min engine would have to output 1.5 times as much energy as it had before, and the 5g/min engine would have to output half that amount (0.75 times as much energy as the 10g/min engine had been putting out).
If the 5g/min engine were added in parallel the other way, half the water pumped by the 10g/min pump would go through the 5g/min engine. This would mean that the constriction would only half to pass half as much water, and would thus only produce half as much back-pressure and dissipate a quarter as much energy as it had with just the 10g engine. The amount of energy the 10g/minute engine had to add to the water would be half of what was necessary when it was operating alone, and the 5g/minute engine would actually extract energy from the water.
The problem with wiring the engines in series is that any water which flows through one must flow through the other; if 10g/minute flows through the upstream engine but only 5g/minute flows through the downstream one, that would imply that 5g/minute must somehow be accumulating in the pipe between the engines. Since water is very slightly compressible, such a thing might be possible for a brief moment, but the more water accumulates there the greater the pressure of the water there. The 10g/minute engine would have to add a very-rapidly increasing amount of energy to the water, and the 5g/minute pump would have to extract almost all of that energy from the water. The amount of energy transfer would rapidly increase until such time as the increased pressure prevents the 10g/minute pump from handling its full 10g/minute, causes the 5g/minute pump to let through more than 5g/minute, or causes the pipe between the pumps to fail.
As with the water in the above example, "electron-stuff" is very slightly "compressible". It doesn't take a very large excess or shortage of electrons, however, to build up a really huge amount of "back-pressure". If electrons are entering something at a rate of 1 amp, they are going to have to leave at the same rate; at a current of 1 amp, even a microseconds' worth of excess electrons would be a lot.