Sanhedrin, Chapter 9, Mishnah 4. The Mishnah begins by stating: "Mi she'nischayev bishtei misos beis din - nidon bachamurah" - one who becomes liable to two death penalties from the court is judged with the more severe one. A person who becomes liable for two different death penalties is judged with the more severe punishment of the two.
Two liabilities for two sins:
A person who worshipped an idol and became liable to stoning, and on his way to execution murdered someone else - the murderer is judged with beheading, which is a lighter punishment. It is obvious that the additional sin does not make it easier for him, for there is no logic that the murder would exempt him from the severe punishment and grant him a lighter one.
The opposite case: A person who was convicted of murder and became liable to beheading, and on his way to execution blasphemed. The punishment for a blasphemer is stoning, the most severe of punishments. One might have thought that this person is already considered a 'dead man walking' - once he is sentenced to death he can no longer become liable to a more severe punishment, for he has already lost his life. But this is a mistake: he is judged and punished even for the second sin, with its more severe punishment.
One act with two prohibitions:
The second part of the Mishnah deals with someone who did not commit two separate sins, but performed one act that contains two prohibitions. One who cohabits with a married woman is punished with strangulation; one who cohabits with his mother-in-law is punished with burning, which is more severe. What is the law for one who cohabits with a woman who is his mother-in-law and is also a married woman, and both prohibitions apply to her at once?
Here too, there is no logic to say that one who cohabits with his unmarried mother-in-law would be liable to the severe punishment, while if she is also married his sentence would be lightened to strangulation alone. The question touches on a different point: a great rule in the entire Torah is that a prohibition does not take effect upon a prohibition - something that is already prohibited is not prohibited a second time, since forbidden is forbidden, and there is only one prohibition here.
From here arises a fundamental dispute in our Mishnah: Is this rule absolute, or does it not apply in a case where the second prohibition is an 'issur mosif' - an adding prohibition that adds an additional layer of prohibition, or an extra stringency in punishment:
The view of the Sages: A prohibition takes effect upon a prohibition. A second prohibition can take effect upon a matter when it is an 'issur mosif', an adding prohibition, which is more severe than the first.
The view of Rabbi Yosi: A prohibition does not take effect upon a prohibition, and the rule is absolute with no exceptions.
The case in the Mishnah:
A married woman who has a daughter, and a man married the daughter. Once he married the daughter, the mother - who was already forbidden to him from the outset as a married woman - also became forbidden to him as his mother-in-law, and this is a second reason for her prohibition. According to Rabbi Yosi, the second prohibition does not take effect, since the woman was already forbidden before he married her daughter and no prohibition was added to her; therefore, the punishment for one who cohabits with her is only strangulation, as it was originally. And according to the Sages, a prohibition is now added to her, and he is even liable to burning.
The opposite case: Reuven who married a woman whose mother is a widow - at the time of his marriage, his mother-in-law was forbidden to him solely as a mother-in-law, and its punishment is burning. If the mother-in-law subsequently married, even Rabbi Yosi admits that she also became a married woman, but the punishment remains burning, since that was the law originally. According to Rabbi Yosi, everything follows the preceding prohibition, and according to the Sages, the prohibitions accumulate, and an additional layer of prohibition takes effect upon the original layer of prohibition.
And so we learned in the Mishnah: One who committed one sin that contains two different punishments is judged with the more severe of them. And Rabbi Yosi disagrees and says that since the rule 'a prohibition does not take effect upon a prohibition' is absolute, the preceding prohibition determines the punishment, and even if a second layer of prohibition was subsequently added, it is meaningless. Therefore, a married woman who subsequently became his mother-in-law, her punishment is strangulation, like any married woman.
In conclusion: The halachah follows the view of the Sages, that an adding prohibition takes effect upon a prohibition and an additional punishment can be added. Therefore, according to the words of the Mishnah, one who becomes liable to two death penalties from the court - whether through two separate sins or through one act that contains two prohibitions - is judged with the more severe of them.