Question:
When do Referees stop the jam-in-progress for a Jammerless Jam? When the second penalized Jammer is sent to the penalty box or when the second penalized Jammer reaches the penalty box?

Answer:
The Referee should stop the jam-in-progress and run the Jammerless Jam procedure when the second penalized Jammer sits in the penalty box, that is when her penalty time starts. One exception: when the first penalized Jammer has 10 seconds or less remaining on her penalty the Referees should not run the Jammerless Jam procedure.  A player in the penalty box should stand when she has 10 seconds remaining on her penalty.