It seems obvious to the gemara that kamapik n'chi riv'a d'mamona apuma d'chad sahada is a clear s'voro. Rashi and the Ran both quote the possuk "al pi sh'nayim eidim" as a source, but what's the mashma'us of that possuk? I would hear (based on a d'rasha given by the Providence Rosh Yeshiva) "sh'nei eidim" as indicating equality within a pair, as in the sh'nei se'irim of Yom Kippur, the sh'nei ham'oros, etc., but not necessarily the same meaning inherent in "sh'nayim".
Marc Diamond, Providence, RI, USA
Actually there is also a Pasuk that says "Al Pi Shnei Eidim" (Devarim 19:15), although this is not the Pasuk that Rashi quotes as you point out.
I do not know why there should be a difference between Shnei and Shenayim, surely Shnei is just the construct form of Shenayim?
I think that this is a Sevara, in other words not a Derashah from the Mashma'us of the verse, rather that Chazal understood each Eid must be of equal significance in the Eidus and therefore when the Torah says Al Pi Shenayim Eidim, it means that each one of these two Eidim must cause half the effect of the Eidus of both Eidim together.
Ha'Gaon Reb Shimon Shkop writes in Sha'arei Yosher (7:1 DH v'Yesod) that Chazal had a tradition (Kabalah) that this Pasuk means to tell us that half of the Eidus is upheld by each individual Eid. However I am not sure that he meant to say that there was a Kabalah that a half Davka, not more or less. He may have just meant to say how the Eidus works (see there).