Typically, there is no issue running the electric pump in series (in line) with the mechanical pump. If either fails the other can still function and fuel flows through both fine, even when the electric is not energized. And, the electric can be selected to run by switch only as needed. There really isn't any reason to parallel connect the pumps as shown in the schematic (which will also work fine). The downside of either configuration is that if the mechanical pump diaphragm ruptures, fuel can potentially flow into the crankcase when the electric pump is in operation. That is true in either case, as shown in the schematic, or the simpler inline configuration. A low pressure 6V pump inline with the mechanical is a good configuration, IMHO.
