You may try Helen and Casey by Arania.
There is a chance that you haven't seen it... but if you don't know Arania's work it kind of means you are not a TF fan lol.
The cow girl touches the normal one and each part of her body that gets touched begins to change until she is like her.
I just assume everyone on this board has seen it since it is just so good even if you are not really a TF fan.
For those who are unfamiliar with her work check her website http://arania.kamiki.net/ For the particular sequence (10 pages long) go to the TF section and go way down. Or just do a find command to make it easier- Helen & Cassie Comic