If R. Eliezer says "Yetaher," the Metzora is Tahor as soon as he gets another Nega, even if the Kohen will not see it for another seven days.
How can a Yisrael decide that the old Nega is Tahor due to the appearance of a new Nega? He is effectively saying that the new rash is indeed a Nega - but we know that only the Kohen may determine what is a Nega!
This is especially difficult according to Rashi, who holds that he is not Tahor retroactively.
Pesach Feldman
When the Gemara states (second line from top of 34b), "Yitaher," it means that he is only Tahor after the Kohen saw it. Even according to the possibility raised by Rav Papa below that it reads "Yitaher" without a Vav (and Rashi, DH Yitaher, writes that according to Rav Papa he is Tahor from the first Nega immediately when the second one appears), this also means that the Kohen must see the second Nega. The Chidush of the "Yitaher" possibility is that he does not have to wait until the second Nega becomes Tahor for him to become Tahor from the first, but he does require a Kohen to say that the second Nega is Tamei.
Kol Tuv,
Dovid Bloom
1. I think that I have support, b'Siyata d'Shmaya, for the above reply from the He'oros of Rav Elyashiv zt'l on Bechoros 34b (page 254). The subject of his paragraph there is, "Is it possible to make Tahor the Matzora without a Kohen?"
Rav Elyashiv explains that even according to the first side of Rav Papa's question, that immediately when another Nega appears he becomes Tahor from the first one (see Rashi DH Yit'har), nevertheless it is necessary that a Kohen pronounce him Tahor because he cannot become Tahor on his own. Rav Elyashiv says that the second Nega's appearance cannot be better than the first Nega healing. Since, in the latter case, a Kohen is required to pronounce him Tahor, when a second Nega appears it is also essential to have a Kohen pronounce the first Nega as Tahor.
2. I found that the Sefas Emes (34b, DH b'Gemara Ba'i Rav Papa) appears to be in doubt about whether it is necessary that the Kohen must rule that the second Nega is Tamei.
The Sefas Emes is commenting on the question of Rav Papa of whether the text is "Yit'har" or "v'Yit'har," and on the question of the Gemara of what difference does it make if it says "Yit'har" or "v'Yit'har." The Sefas Emes asks that it is difficult to understand why the Gemara is doubtful about the difference between "Yit'har" and "v'Yit'har," because there appears to be a very clear difference, as follows:
If we say that as long as a second Nega develops, even if the Kohen did not say it was Tamei when he saw it (for example, the Nega had already shrunk to less than the size of a barley grain by the time the Kohen saw it, in which case it could not be pronounced as Tamei), nevertheless the first Nega is still rendered Tahor by the appearance of the second Nega, then it follows that there is a significance difference between "Yit'har" and "v'Yit'har." According to the text of "Yit'har," the first Nega would become Tahor even if the second Nega was less than a barley grain by the time the Kohen saw it. According to the text of "v'Yit'har," we require the Kohen to pronounce the second Nega as Tamei and that the second Nega should later heal.
3. We see that the Sefas Emes entertains the possibility that it is not necessary for the Kohen to see the second Nega in order to make it Tamei. Possibly, we may explain the reason for this as follows. First, we should be aware of the fact that the discussion in our Gemara -- concerning the person who cut off the Baheres -- is entirely occupied with Tum'ah d'Rabanan, because mid'Oraisa -- since the Baheres has been removed -- it is no longer Tamei. The entire Sugya is about a penalty that Chazal gave the Metzora for cutting off the Baheres.
Since the Tum'ah involved here is only mid'Rabanan, we may now say that Chazal were more lenient about how the Metzora becomes Tahor from it. It is sufficent that we should know that the Kohen developed any kind of Nega and then became Tahor from that Nega. Once we know that the Kohen was healed from the second Nega, we can then apply the logic written by Rashi (on the third line of 34b) that when the second Nega is cured, we may now say that if the first Nega would still have been present it would also have healed. The reason for this is given by the Bartenura in Nega'im (7:4, DH k'she'Yivaled) that since he was healed of the second Nega, this shows us that Hashem had mercy on the Metzora, and even if the first Nega would not have been cut off, nevertheless Hashem would have had mercy on him and healed this Nega too.
Kol Tuv,
Dovid Bloom