Found this quote from an earlier paper on TJI. It explains the benefit of the extra spark plug external to the pre-chamber.
Additionally, a new concept in “Spark Assisted Jet Ignition” combustion was proposed to enable stoichiometric boosted high load operation, where the jet igniter is coupled with a conventional spark plug. With this concept, some of the main chamber charge is consumed first using the conventional spark ignition flame propagation process, with the remaining charge consumed by the jet ignition process Figure 14. Two possible configurations for “Spark Assisted Jet Ignition”, where some of the main chamber charge is consumed using the conventional spark ignition flame propagation process and the remainder using jet ignition combustion in order to increase the burn duration and reduce the heat release at stoichiometric boosted high loads.
A Gasoline Fueled Pre-Chamber Jet Ignition Combustion System at Unthrottled Conditions
Attard et al / SAE Int. J. Engines / Volume 5, Issue 2(May 2012) 327