1) The gemara 2a is referring to a scenario where the husband warned his wife not be alone with a suspect individual, but she was alone with him and one witness testified that they had relations. Here, one witness is believed because there is "Raglyaim LeDavar"; there is a good basis to believe the one Eid because of the Kinuy and Setirah that preceeded the testimony of this one witness.

2) The gemara 7a refers to a scenario where the husband is taking his wife to Yerushalayim because he suspects her of being unfaithful. On the other hand, since they are husband and wife there is a possibility that they may have marital relations, but this is more unlikely since the husband is taking her to the Beis Hamikdash with the possibility that me might drink the waters and be proved to have been mezaneh. Since we obviously had no Kinuy and Setirah in the latter scenario; and since the husband knows that his wife is forbidden to him until it is proved that she was not unfaithful; one witness is not sufficient to prove that there was Biyah. 1 witness is only sufficient where there is Raglayim LeDavar, as on 2a, whilst in 7a there is no Raglayim LeDavar so 2 eidim are required.

Wishing you a Healthy Summer

Dovid Bloom

Follow-up reply:

I found, bs'd, that the sefer Dvar Shaul on Masechet Sotah (by Rav Shaul Kososvsky-Shachor) 10:3, asks this question.

1) Firstly he points out that it is only if the husband denies that he had relations with his wife whilst on the journey that one Eid is not believed, but if one Eid testified that they had been together and the husband made no comment, this would be sufficient to prevent the drinking.

2) Dvar Shaul explains that if the husband denies it, then separating the husband from the wife is considered a "Davar She-Be-Ervah" and there is a major rule (see Gitin 2b) "Ein Davar She-b'Ervah Pachot MiShtaim"; one cannot effect anything in matters of marriage and divorce with less than 2 witnesses. If we prevent the drinking on the basis of 1 witness, this means the husband and wife must part, so this is equivalent to giving a Get with only 1 witness, which is impossible.

3) Dvar Shaul writes that it is only when the eid testifies on tumah that the Torah believes him, but if he testifies on relations between husband and wife (which is not tumah but it is a Torah prohibition since the wife is suspected of zenus) this is considered as testimony on a side factor that prevents her drinking, and the Torah does not believe 1 eid for this.

4) In short, the Dvar Shaul answers that the Torah only believes 1 eid about tumah, but not on a side issue.

KOL TUV

Dovid Bloom