Rav maintains that two sources are necessary to teach us that the Kohen burning the Parah Adumah may not remove his attention from what he is doing -- one for the Shechitah and one for the Sereifah. The Gemara goes on to state, however, that there is another source for this Halachah that applies to all aspects of the Parah Adumah except those that are only tangentially related; namely, the addition of certain ingredients to the burning cow. Why, then, are two sources necessary according to Rav?
Moshe Binyamin cohen, Yerushalayim
Tosfos (42b, DH "l'Mishmeres") basically asks your question. He answers that even with a general Pasuk stating that the Avodos done with the Parah Adumah cannot have Hesech ha'Da'as, we would think that Shechitah and Sereifah are not included. As Shechitah is applicable even by Chulin (regular non-Korban meat), we would think this is not the type of Avodah which requires one not to be Mesi'ach Da'as. Similarly, Sereifah is an Avodah which does not immediately produce something which can be sprinkled as does Shechitah (the blood of the Parah) and the gathering of the ashes etc.. This is why Shechitah and Sereifah needed separate Pesukim (see Tosfos further regarding why everything else could not have been learned from the Pesukim of Shechitah and Sereifah).
Kol Tuv,
Yaakov Montrose