Bifurcation of the aerosol trails is due to the duel thrust emissions from the high-bypass turbofan engine - with a minority of thrust coming from the combustion stage and most thrust developed in the bypass stage.
Only 15% to 20% of the air intake goes into the engine combustion chamber to produce the water vapor necessary for a contrail to form. Remarkably, a whopping 80% to 85% of the thrust that exits the engine NEVER passes into the combustion chamber. This means that 85% of the thrust exiting the engine is INCAPABLE of producing a contrail. Furthermore, the 15 to 20% that is subjected to combustion is almost completely neutralized by the dominant mixing from the 85% of "dry" thrust that passes around the combustion chamber to provide almost all the thrust. The thrust from the combustion chamber creates the appearance of a "bifurcated" trail because the 15% of the emission creates the small doughnut hole in the bypass thrust where the aerosols are injected, hidden beneath the engine cowling. This finding explains why no fuel additives are found to explain the chemical trails since the additives would degrade engine performance if passed though the combustion chamber. To avoid engine damage, the aerosols are injected in the bypass stage where aerosols will not come into contact with critical engine parts.
Lastly, commercial pilots likely have little control of aerosol emissions since the ability to start and stop the chemical spray is controlled remotely as in "drone" or fly-by-wire technology from a remote site. Modern jet engines are fitted with transponders that report engine performance to the manufacture through transmission of data to a satellite. With radio frequency communication already in place it becomes possible for the engine to respond to a remote satellite command where chemical aerosol emissions are turned on and off automatically without intervention by pilot or crew. This explains why pilots may not be good whistleblowers since they have no, or insufficient knowledge of the secret devices located inside the aircraft that supplies the engine.