Answer

More power to you,
I mainly wanted to examine the formal issue of why we do not say two migos. Note well: in the Talmudic passage in Bava Metzia ("one who rides on an animal"), in the dispute between Rabbi Eliezer and the Sages, Ulla (who explains the dispute) does not argue that the wealthy person cannot declare his property ownerless, as some of the later authorities you mentioned hold. He simply argues that according to Rabbi Eliezer, we do not say two migos. That is, there seems to be some kind of formal rule here, and clearly it has roots connected to how we understand the concept of migo. I interpret it simply—and presumably you would not disagree—that this is a case of one migo after another, that is, in sequence. After the wealthy person is permitted to take gleanings because of the migo that he could declare his property ownerless, we accept that result as a given, as an input for the continuation of the process, and then apply another migo from one poor person to another, which everyone accepts. According to this, the Sages do not reject the first migo in and of itself, but only when it serves as data for the continuation of the process (two migos). It may be that the matter is connected to the legal nature of migo: whether the essence of the claim is connected to the credibility of the claimant or to the strength of the claim (in Kovetz Shiurim he holds that this is the dispute between Rabbah and Rav Yosef). If we say that migo strengthens the claimant’s power, then in a case of two migos he is making double use of the migo, and that is why the Sages object.
 In terms of the hermeneutical principles, this is reminiscent of the idea that the result of one exposition becomes the starting point for another exposition (a derived law from a derived law, the famous passage in Zevachim).
In logical terms, it may be similar to probability upon probability, where the final result is obtained by multiplying the probabilities, which greatly reduces the final probability.
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Questioner:

More power to you,
I mainly wanted to examine the formal issue of why we do not say two migos. Note well: in the Talmudic passage in Bava Metzia ("one who rides on an animal"), in the dispute between Rabbi Eliezer and the Sages, Ulla (who explains the dispute) does not argue that the wealthy person cannot declare his property ownerless, as some of the later authorities you mentioned hold. He simply argues that according to Rabbi Eliezer, we do not say two migos. That is, there seems to be some kind of formal rule here, and clearly it has roots connected to how we understand the concept of migo. I interpret it simply—and presumably you would not disagree—that this is a case of one migo after another, that is, in sequence. After the wealthy person is permitted to take gleanings because of the migo that he could declare his property ownerless, we accept that result as a given, as an input for the continuation of the process, and then apply another migo from one poor person to another, which everyone accepts. According to this, the Sages do not reject the first migo in and of itself, but only when it serves as data for the continuation of the process (two migos). It may be that the matter is connected to the legal nature of migo: whether the essence of the claim is connected to the credibility of the claimant or to the strength of the claim (in Kovetz Shiurim he holds that this is the dispute between Rabbah and Rav Yosef). If we say that migo strengthens the claimant’s power, then in a case of two migos he is making double use of the migo, and that is why the Sages object.
 In terms of the hermeneutical principles, this is reminiscent of the idea that the result of one exposition becomes the starting point for another exposition (a derived law from a derived law, the famous passage in Zevachim).
In logical terms, it may be similar to probability upon probability, where the final result is obtained by multiplying the probabilities, which greatly reduces the final probability.

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Rabbi:
I didn’t understand. The migo being discussed here is not an evidentiary migo ("why would I lie?") but the power of "it is in his power." The problem is not the first migo but the second. The migo cannot take effect unless the previous migo has already actually been applied, as I explained. There is some similarity to sorites or to a derived law from a derived law, but that kind of similarity is not an explanation. As for multiplication of probabilities, it is somewhat similar, although we are not talking about probability but about power. Suppose the power given by the first migo is 80%, and the chain with the second has a force that is multiplied by the first: 0.64.