This post is dedicated to the complete and rapid healing of Rabbi Chanan Feld, HaRav Chanan Velvel Simcha Ben Bryna.
When we put these two laws together and add jealousy into the mix, it gets complicated! Fasten your seatbelts...
Consider a case when the husband and Wife A travel abroad, and leave Wife B home alone. If Wife A returns alone and reports her husband's death, we might think her testimony is enough to make both wives eligible to re-marry.
However, this is not the case. Wife A, whens she testifies about her husband's death, does not enable Wife B to remarry as well. The Rabbis feared that hatred between the husband's two wives might be strong enough that the Wife A might want to cause the Wife B to enter an illegitimate new marriage by lying about their husband's death. Wife A could then have her husband to herself.
One might think that we would believe Wife A's testimony and permit Wife B to remarry if Wife A also remarries, because in such a case if Wife A is lying, neither Wife A or Wife B can remain with their original husband. However, the Rabbis feared that hatred between the wives might be so strong that Wife A would be happy to sacrifice her marriage to the husband as long as it destroyed Wife B's marriage as well.
The Rabbis draw a parallel to the story of Samson's death. Just before his death, Samson was blinded and being readied for execution by the Philistines in their temple. He held two supporting columns for the temple, and prayed that his strength could be recovered for a moment, so that he could take down the columns, causing his own death but also that of all of his enemies under their temple's roof. Here's the JPS translation: He said (Judges 16:30), 'Let me die with the Philistines!' and he pulled with all of his might. The temple came crashing down on the lords and on all the people in it. Those who were slain by him as he died outnumbered those who had been slain by him as he lived. The Rabbi's feared that Wife A might also cry out 'Let me die with the Philistines,' and happily bring town the temple of her marriage upon her own head as long as it destroyed that of her enemy Wife B.