Davar V'lo Chatzi Davar

The Rishonim on daf 70b in Baba Kama discuss the halacha of davar v'lo chatzi davar. This halacha states that when eidim testify on something they must testify on "a whole thing" and not "a half thing". The gemara says within the shitas Rabbanan that the halacha applies in certain cases and not in others. For example, it takes three years of living on a piece of land for someone to make a chazaka on that land. Once he makes a chazaka the land is presumed to be his. This is the halacha of shnei chazaka. The gemara says that if three sets of eidim come and each set testifies that the baal din has been living on the land for one year, the three sets of eidus are mitztaref and it is not a problem of chatzi davar. The gemara gives then gives a contrasting case. In order for a woman to be a gedolah she must display two hairs as a sign of maturity. Now, if two sets of eidim come and one testifies about one hair and one the other, that is a problem of chatzi davar. The gemara explains that in this case each set of eidim is actually only testifying that the girl is a ktana. Thus, the eidus cannot be mitztaref to make her a gedolah. This is the case of achas b'kreisa v'achas b'gabba. The Rishonim all try to figure out what precisely is the difference between the case of shnei chazaka and achas b'gabba v'achas b'kreisa.

Here is a rundown of the basic shittos:

Rashi: By the case of achas b'gabba each one is in the end testifying that she is a k'tana, so it's chatzi davar. However, in the case of shnei chazaka each group is testifying that the person currently on the land is a muchzak. Thus, we can add the eidus together to give the baal din a chazaka.
R' Chaim in M'Ginzei HaGrach Siman 54 explains Rashi along the lines of the shitah of the Ramban in Baba Basra. That shitah is that really when someone is on a piece of land even for a minute he makes a chazaka. Why then is three years needed? The three years are needed because in every chazaka there is a reyusa (a deficiency) of the missing shtar. If this baal din really is the owner why doesn't he have a shtar? Once three years go by there is good reason for him to have lost his shtar and the original chazaka kicks in.
Acc. to this, Rashi is saying that each set of eidim are in fact testifying to a chazaka. It's only a reyusa that forces us to be mitztaref the three sets. This, therefore is still considered a "full" davar.

Rif: The Rif answers that by shnei chazaka the eidus is considered complete because even one set of eidus could be effective with regards to making the baal din have to pay for any peiros he ate from the field should the field turn out not to be his. So, even though the eidus is not effective independently for a chazaka, since it can affect other dinim it is not a problem of chatzi davar.

Tosafos: By shnei chazaka the eidim saw everything that was possible to be seen within that time frame. This makes it a complete eidus. However, by the case of gadlus the eidim could've seen both hairs, thus rendering their eidus incomplete and pasul.

Shita Mekubetzes (Baba Basra 56b): This answer is also mentioned in entry 2 in the Kobetz Shiurim on Pesachim. The answer is that by gadlus the hairs themselves are the gadlus (and not a mere siman for the gadlus), whereas by chazaka the years of chazaka are just a raaya that the land is his, but they don't make the land his. The idea here is that if the saaros are actual gadlus testifying on only one of them is considered testimony on a chatzi davar. This is a classic utilization of the sibah/siman chakirah which we recently mentioned here. This answer is quite puzzling because saaros of gadlus are actually referred to as simanim. Yet, here we are anwering that they, in fact, are not a siman, but a sibah! (Before you argue on this claim of mine, please read on.)

Ohr Sameach (first entry in Hilchos Sotah): This answer completely reverses the logic of the last one! Simanim are, in fact, a siman, not a sibah (as I would argue the name implies!). The Ohr Sameach proves this from the fact that there are cases where one becomes a gadol without them (like by a saris or if one has children). Therefore, since it is only a siman it is considered chatzi davar. The Ohr Sameach continues that shnei chazaka are also a siman, and that's why the Rif had to come up with his answer about the peiros. Had either of these cases been a sibah, no answer would have been necessary.

There are other answers to this question as well.

This example is a good illustration of some of the downsides of the sibah/siman chakirah:

1. The first issue here is that the achronim can't seem to agree on whether simanei gadlus are considered a siman or a sibah. What I think we should really ask ourselves is whether this is at all a provable chakirah!? The Ohr Sameach proves that they are a siman because it is possible to have gadlus without them. Is that a proof? Maybe there are different causes of gadlus? The siman/siba chakira is basically asking whether something is a cause of something or just a correlation. Are the hairs the actual gadlus or just a siman for gadlus? All we know is that the Torah says that when a woman displays these hairs she is legally considered a gedolah. She has a halachic change of status. It's not a scientific assertion that we are dealing with over here that we should be asking if it's cause and effect or not. It's a legal and halachic issue. The hairs cause us to change her status from a ktana to a gedolah. The issue of siman/sibah seems to belong more to the scientific realm.

2. The relevance of the sibah/siman chakirah here is very vague. Let's isolate the case of shnei chazakah. Remember, we are trying to determine why when one set of eidim testify to the fact that the baal din has been here for one year that it is considered a "full" eidus and not chatzi davar. So we are looking for some aspect of their eidus that is "complete". Rashi/Rav Chaim give us that in the fact that their eidus makes the guy "a muchzak". The Rif also renders the eidus complete in terms of the peiros. (Tosafos goes in a slightly different direction by giving us a complete reeyas haeidus within a time frame.)
However, when we move to the siba/siman chakira we seem to lose the direction. For example, in the Shitah/R' Elchonon the eidus is supposed to be complete because it is a siman. Why? For what specific issue has the eidus completed anything? What legal change did the eidus bring about? To make it even more confusing, the Ohr Sameach goes the other way - the eidus is complete davka because it is not a siman but a siba for the chazaka (if it would be true that it were a siba). Again, why? If it's a siba but it doesn't actually complete any chazaka why should it not be chatzi davar?

I don't know if these arguments can really overturn all the applications of the siman/sibah chakirah. That's not really even my goal here. However, intuitively I have almost always been bothered by this chakirah so I thought it would be worthwhile to try to concretize some of what's bothered me about it. If you can enlighten me on the subject I would appreciate your input.