(a) That is an excellent point. It seems that the most simple, textual approach is that since the Mishnah has made no mention of the wife of the Kohen Gadol until now, it is not fitting to write that a second woman was prepared "Tachteha," in her place, since there was no prior reference to her. Rather, the Mishnah should have said "Tachas Ishto," but if so it would have had to add two words and the Mishnah avoids such superfluous additions when possible. In contrast, when discussing the substitute for the Kohen Gadol, since it made mention of the Kohen Gadol earlier in the Mishnah, it is fitting to say "Tachtav," and adding one word is not considered unnecessarily lengthening a Mishnah.

(b) In a deeper sense, we can explain the Mishnah as follows. The Kohen that was appointed as a back-up for the Kohen Gadol certainly had to learn all of the Halachos of the Yom Kipur Avodah and had to become familiar with all the different animals and procedures. In contrast, the woman appointed as a back-up wife for the Kohen Gadol had no responsibilities whatsoever. Therefore, regarding the substitute for the Kohen Gadol, the Mishnah says he was appointed "Tachtav," meaning that he would not only take his position, but he would also take his responsibilities of doing the Avodah.

(c) Alternatively, it could be that the second wife was not merely "in place of" (Tachas) his original wife if she died. Rather, the Gemara explains that the Kohen Gadol would divorce his first wife conditionally in such a way that if she were to die in the middle of the day, it would not be that l'Mafrei'a he was married to two women from the start of the Avodah. Consequently, the second wife was not only his wife from the time that the first one died, but she was his wife before the first one died, from the beginning of the day! Therefore, the term "Tachteha" is inappropriate.

M. Kornfeld