Комментарий к Охолот 2:6

השדרה והגולגולת משני מתים – half of the vertebrae of the spine from this corpse and half of the vertebrae of the spine from another corpse, the skull.

A backbone or a skull [made up from the bones] of two corpses,
A quarter [of a log] of blood from two corpses,
A quarter [of a kav] of bones from two corpses,
A limb of a corpse from two corpses,
And a limb [severed] from a living person, [such a limb being made up] from two persons,
Rabbi Akiva declares [the all] unclean
But the sages declare them clean.
In mishnah two we learned that Rabbi Akiva holds that a quarter of a log of blood mixed from two corpses is impure, even though there is not the requisite amount of blood from one corpse.
In all of these cases we have the amount of corpse stuff required to transmit impurity, but it is composed (or should I say, decomposed) of material from two different corpses. To be honest, I don't know how one puts together a limb that comes from two different people, whether it has to be attached or not. It seems like the two limbs are placed together such that they look as if they are attached.
Rabbi Akiva says that in all of these case, the limb, blood or bones defiles in the same way that a limb, blood or bones from one corpse does by contact, carriage and overshadowing.
The other rabbis say that these things are clean. What this means is that they do not defile through overshadowing. However, they do defile through contact and carriage, for even a bone the size of a barley-corn defiles in these manners.

ואבר מן החי משני אנשים – half of a limb that is torn off from this living person and half of a limb that is torn off rom another living person, and they appear as one limb.

וחכמים מטהרין -from defilement by overshadowing in a tent. But they defile through contact and carrying because of a bone the size of a barleycorn. And the Halakha is according to the Sages.