What is the ruling if two men testify that two others committed murder, and subsequently, the two accused men testify that the first two men committed the murder, and the heir validates the first pair while invalidating the second pair?

Primary text

If two witnesses testify that two other men committed a murder, and those two accused men then testify that the first two witnesses were the murderers, and the heir confirms the testimony of the first two and invalidates the testimony of the second two, execution is obligatory upon the two later testifiers. This is because the heir invalidates their testimony, and they are defending themselves against harm through their testimony.