The Gemara asks what the difference is between giving Malkus to two Edim Zomemim (in which case each one receives the full amount of 39 Malkus), and giving a monetary fine to Edim Zomemim (in which case the entire sum is divided among them). Rava answers that if the Malkus is divided among them, then the requirement of "Ka'asher Zamam" is not fulfilled, but if the monetary fine is divided among them, then the requirement of "Ka'asher Zamam" is fulfilled. This is because "Mamon Mitztaref, Malkus Lo Mitztaref."
How, though, does this answer the question? Why can we not join together the Malkus of each witness like the Mamon of each witness is joined?
1. The Beis ha'Levi (end of Parshas Beshalach, DH Cherpah) writes that Malkus given to one witness does not join the Malkus given to another since any subsequent Malkus that a person receives is more painful than the first ones, since his flesh is already sore from the first ones. Therefore, if the 39 Malkus would be split between the two witnesses, neither one of them would experience the full pain that they wanted to inflict upon the defendant. The defendant would have received the full 39 Malkus, of which the second half would have hurt much more than the first half. Thus, each witness must receive the full amount of Malkus that they tried to inflict, in order that "Ka'asher Zamam" be fulfilled. This reasoning does not apply to a monetary (non-corporal) punishment, though.
However, the answer of the Beis ha'Levi seems problematic for a number of reasons. First, certainly a poor person feels the pain of the loss of a larger sum of money, and thus having to pay the additional 100, after already paying 100, causes greater pain. Thus, each witness should have to pay the entire amount so that "Ka'asher Zamam" is fulfilled!
Second, when a wealthy man testifies (falsely) against a poor man, we should make the wealthy man pay all of his money, until he is left with the same amount that the poor man would have been left with, in order to fulfill "Ka'asher Zamam"! Similarly, when administering Malkus to Edim Zomemim, how will "Ka'asher Zamam" be fulfilled when a strong, healthy person testifies falsely against a weak, sickly person? The pain that each one experiences as a result of the Malkus is not the same!
Third, why does each witness receive the full amount of 39 Malkus? It should suffice if each witness is given half of the pain that they wanted to inflict on the defendant, and the pain that each one experiences should join, just like the Mamon joins.
2. It must be that we do not measure one person's pain against another person's. Rather, the intention of the Gemara is, as Rashi explains, that Mamon joins since it is all given into one hand (that is, it is all given to the defendant).
The Gemara's intention in differentiating between Mamon and Malkus is clear: with Mamon, the purpose of the payment is to compensate the loss of the one to whom the payment is being made. Therefore, as long as the person receives the entire sum of money, even if it is being given to him by ten different witnesses, the requirement of "Ka'asher Zamam" is fulfilled, and each witness fulfills his obligation. In contrast, with Malkus, the defendant receives no benefit from the Malkus that the witnesses receive. The Malkus is given not to compensate the defendant, but to punish the witnesses, and therefore "Ka'asher Zamam" must be fulfilled with each witness individually. (This is the intention of Rashi here.)
However, there still seems to be a problem with this answer. This answer makes sense according to the opinion that maintains that "Edim Zomemim Mamona" -- since the witnesses tried to cause a loss to the defendant with their testimony, "Ka'asher Zamam" is fulfilled when the defendant receives all of the money that they wanted to make him lose. (We have addressed elsewhere why it should be called Mamona if the witnesses did not actually cause a loss to the defendant; see also Tosfos to Bava Kama 4b DH v'Edim.)
However, according to the opinion that maintains that "Edim Zomemim Kenasa," the punishment that the witnesses receive is not a compensatory requirement for the sake of the defendant, but rather it is a punishment given to the witnesses for the crime they committed, similar to punishments for any other Aveirah. Accordingly, logic would dictate that each witness should receive the full amount of the punishment, whether it is Malkus or a Kenas! Because of this question, RAV AZRIEL HILDSHEIMER (end of YD I 235) writes that our Mishnah must be following the view that "Edim Zomemim Mamona" -- but this is a very implausible view.
It appears, therefore, that even according to the opinion that "Edim Zomemim Kenasa," the Kenas is that the witness is to be considered as though he successfully inflicted the damage to the defendant through his testimony, and therefore he must pay. Accordingly, the payment that he makes (that is, after Beis Din has issued the verdict requiring one to pay) is identical to the payment made in every case of a Chiyuv Mamon . The Kenas is that it is necessary for him to pay as if he had actually caused damage.
This understanding of the Kenas of Edim Zomemim is also evident from the fact that the Edim Zomemim are required to pay to the defendant . If their payment is simply a Kenas, then why should they pay the Kenas to the defendant? They should pay it to Beis Din and not to the defendant! It must be that, as we have explained, the Kenas is that they are to be considered as though they caused damage to the defendant and they must pay him for the damage they caused. This logic was obvious to the Gemara (and that is why it cites no source for the Halachah that Edim Zomemim pay directly to the defendant; the Acharonim discuss this question at length -- see Teshuvos Radbaz and others), and this is indeed the logic behind all cases of Kenas in the Torah -- the Kenas is that we view the perpetrator as though he actually caused damage to the victim.
If so, it is clear that even if the payment of Edim Zomemim is a Kenas, once the victim has received the entire amount of the Kenas, it is no longer necessary to have the witnesses pay any more money. Hence, "Mamona Mitztaref."