Orlah Chapter 2, Mishnah 17. This mishnah too deals with the meat of offerings, and with a case in which two different types of offerings became mixed together and were cooked along with chullin meat.
The two types of meat before us:
Meat of kodshei kodashim - eaten only by kohanim.
Meat of kodashim kalim - eaten by any Jew, provided he is tahor; one who is tamei may not eat sacrificial meat.
These two types of meat became mixed with one another, and were cooked together with 'basar ta'avah' - ordinary chullin meat, eaten simply for a person's own enjoyment. In this case the taste of the two types of kodashim is discernible in the mixture only when they combine together, and neither one alone is enough for its taste to be discernible.
The words of the mishnah:
"Besar kodshei kodashim u'vesar kodashim kalim shenisbashlu im besar hata'avah - asur l'tamei u'mutar l'tahor" - meat of kodshei kodashim and meat of kodashim kalim that were cooked together with basar ta'avah: it is forbidden to one who is tamei and permitted to one who is tahor.
"Asur l'tamei" - forbidden to one who is tamei:
Even though basar ta'avah on its own is permitted to be eaten by someone who is tamei, both the meat of kodshei kodashim and the meat of kodashim kalim are forbidden to a tamei person. Therefore the two prohibitions combine together and render the mixture forbidden to him.
"U'mutar l'tahor" - and permitted to one who is tahor:
A tahor person who is not a kohen may not eat kodshei kodashim, but the mixture does not contain enough kodshei kodashim for their taste to be discernible on its own; and kodashim kalim are permitted to him. It emerges that the mixture is permitted to him, even though he generally may not eat kodshei kodashim.
Why Rabbi Shimon does not disagree here:
The special novelty of this mishnah is that Rabbi Shimon does not disagree. In all the previous mishnayos Rabbi Shimon held that two different types of prohibition do not combine with one another, and here kodshei kodashim and kodashim kalim do combine to forbid the mixture to a person who is tamei.
The explanation is that with regard to a tamei person these are not two prohibitions but literally one prohibition: a tamei person may not eat any type of offering at all, and his prohibition is identical whether for kodshei kodashim or for kodashim kalim. True, there is a separate prohibition of a zar (a non-kohen) - that a zar may eat certain types of offerings but may not eat kodshei kodashim - but the prohibition that combines here is the prohibition of tumah, and that is a single prohibition. For this reason Rabbi Shimon does not disagree in this mishnah.
In summary: in this mishnah we learned that a mixture of meat of kodshei kodashim and meat of kodashim kalim that was cooked with basar ta'avah is forbidden to one who is tamei, since for him the two prohibitions combine into a single prohibition of tumah; and it is permitted to one who is tahor, since the kodshei kodashim alone are not present in an amount whose taste is discernible, and kodashim kalim are permitted to him. And here, because we are dealing with one prohibition and not with two, even Rabbi Shimon agrees that they combine.