The gemara says that money can be split 3 ways between the עדים זוממים each paying a third while מלקות cannot be split between them, each one gets 39. The question is why not?
The Beis Halevi Al Hatorah at the end of Parshas Beshalach has a very interesting p'shat on this. He explains as follows. When it comes to money, $100 is $100 coming from 1 person or 10. The witnesses as a group tried to cause him a loss of $100, k'aasher zamam is for them as a group to pay him $100. They as a group are experiencing the exact same loss that they wanted to cause him. However, by מלקות while 13 + 13 + 13 = 39 it is not the equivalent of 39 מלקות. The Beis Halevi points out that the 14th maka hurts more then the 13th simply because 13 מלקות preceded it, and so on. The 39th is much more painful then the 13th because it was preceded by 38 מלקות. Therefore, if we split up the מלקות the witnesses may get the same number but do not suffer the same pain and suffering that the person would have suffered. He would have felt the pain of מלקות 14 - 39 which is worse then the pain from any of the מלקות that the witnesses are getting. The witnesses are therefore not experiencing the same pain and suffering that the defendant would have suffered. Because of this (the pain and suffering of the witnesses does not add up to the same pain and suffering that the person would have undergone), splitting up the מלקות it is not called כאשר זמם, they would not be receiving what he would have received but something much less (even though a total of 39 מלקות are given) and therefore
they each need to get 39 מלקות.