1) MARRIED TO TWO WIVES IN TWO PLACES
QUESTION: The Gemara relates that when Rav and Rav Nachman (and presumably other Amora'im) traveled to distant towns, they would marry other wives in those towns as a way of preventing any nocturnal mishaps from occurring.
The Gemara asks that marrying a wife in the new town will not help to prevent a nocturnal mishap, because a newly married man and woman are not be permitted to be with each other for at least seven days; the excitement of the marriage causes the woman to have a flow of "Dam Chimud" and become a Nidah right away. The Gemara answers that the Amora'im either informed the women whom they intended to marry seven days earlier, or that they did not actually consummate the marriage with them, but merely performed Yichud with them, and thus the psychological effect of having "Pas b'Salo" ("bread available in his basket") prevented nocturnal mishaps.
According to the Gemara's second answer, how can the woman be considered "Pas b'Salo" if she is forbidden to him because of "Dam Chimud"? (TOSFOS DH Yechudi)
(a) RASHI (as explained by TOSFOS to Yoma 18b) and the RAMBAM (Hilchos Isurei Bi'ah 11:10) write that the Gemara means "Pas b'Salo l'Achar Zeman," she will become permitted to him after seven days. This is called "Pas b'Salo" because he knows that she will be permitted to him in a matter of days. (That is, she is "available" to him as far as the prohibition of being alone with an unmarried woman is concerned, even though she is not available to him as far as the prohibition of Nidah is concerned. Since a Nidah normally becomes permitted in a matter of days, she is called "Pas b'Salo.")
(b) TOSFOS and other Rishonim explain that the Amora'im were only "Tove'a l'Yichud" -- they asked the women to marry them only in order to perform Yichud with them after marriage, but not to have marital relations. Since the women were informed that the purpose of the marriage was only for Yichud, they had no expectations of intimacy, and thus they did not have a flow of "Dam Chimud." (Moreover, if the Amora'im later decided to consummate the marriage, there would be no "Dam Chimud" at that time because the women were already married to them. "Dam Chimud" occurs only when a marriage proposal is offered to a woman who is not married.)
This explanation also answers the previous question of the Gemara. The Gemara asked that one is not permitted to have two wives in two different places lest it lead to problems of Mamzerus. Since the Amora did only Yichud with the second woman, there was no concern that problems of Mamzerus would arise. If he later would decide to consummate the marriage, he would bring her to his hometown.
(c) The RI HA'LAVAN and RA'AVAD (Hilchos Isurei Bi'ah 11:10) explain that the word "Yichud" here does not mean "isolate" themselves. Rather, it means "set aside" for themselves. The Amora did not ask a woman to marry him, but to "set herself aside" for marriage and to make herself available in case he would decide to marry her. This was considered "Pas b'Salo" since the woman was ready to marry him at any moment.
There is no problem of "Dam Chimud" in such a situation for reasons similar to those suggested by Tosfos. A man's request that a woman "be ready" for marriage is not a formal marriage proposal, and thus there is no "Dam Chimud." When he later proposes to her and marries her, she will not have "Dam Chimud" because the proposal was expected and did not take her by surprise. (This approach also answers the earlier question of the Gemara, as described in (b) above.) (See also Insights to Yoma 18:3.)