Yevamot

— Since she is required to perform halizah it is sufficiently known that it  is a mere restriction.  If so, let him, in the case of divorce also,  state it,  and require her to perform halizah, and it will be sufficiently known that it  Was a mere restriction!  — Were you to say that she was to perform halizah it might also be assumed that she may be taken in levirate marriage.  But here also,  were you to say that she is to perform halizah, she might also be taken in levirate marriage!  -Well, let her be taken in levirate marriage and it will not matter at all since thereby she only retains her former status. Abaye raised the following objection against him:  If the house collapsed upon him  and upon his brother's daughter.  and it is not known which of them had died first, her rival must perform halizah  but may not contract the levirate marriage.  But why? Here also it may be said, 'This woman finds herself in the status of permissibility to all,  would you forbid her [marriage on the basis] of a doubt? You must not forbid her [on the basis] of a doubt'! And should you suggest that here also the prohibition is due to a restriction, [it may be retorted that] it is a restriction which may result in a relaxation, for should you say that she is to perform the halizah she might also be taken in levirate marriage! — In respect of divorce which is of frequent occurrence the Rabbis enacted a preventive measure;  in respect of the collapse of a house which is not of frequent occurrence the Rabbis did not enact any preventive measure. Or else:  In the case of divorce, where the forbidden relative is demonstrably alive, were her rival to be required to perform halizah, it might have been thought that the Rabbis  had ascertained that the letter of divorce was a valid document,  and the rival might, therefore, be taken in levirate marriage. In the case of a house that has collapsed. however, could the Rabbis  have ascertained [who was first killed] in the ruin! Have we not learned a similar law in the case of divorce? Surely we learned: If she  stood in a public domain, and he  threw it  to her, she is divorced if it fell nearer to her; but if nearer to him she is not divorced. If it was equidistant,  she is divorced and not divorced. And when it was asked, 'What is the practical effect of this',  [the reply was] that if he was a priest she is forbidden to him;  and if she is a forbidden relative, her rival must perform the halizah.  We do not say, however, that were you to rule that she must perform halizah she might also be taken in levirate marriage!  -Concerning this statement, surely, it was said: Both Rabbah and R. Joseph maintain that here we are dealing with two groups of witnesses, one of which declare that it  was nearer to her and the other declares that it  was nearer to him, which creates a doubt involving a Pentateuchal [prohibition] — 29 Our Mishnah, however, speaks of one group.  where the doubt involved is only Rabbinical. Whence is it proved that our Mishnah speaks of one group? — On analogy with betrothal:  As in betrothal only one group is involved so also in divorce  one group only could be involved. Whence is it known that in betrothal itself only one group is involved? Is it not possible that it involves two groups of witnesses! — If two groups of witnesses had been involved, she would have been allowed to contract the levirate marriage, and no wrong would have been done.  Witnesses stand and declare that it  was nearer to her,  and you say that she may be taken in levirate marriage and no wrong will be done!  Furthermore, even where two groups of witnesses are involved the doubt is only Rabbinical, since it might be said 'Put one pair against the other and let the woman retain her original status'!  This indeed is similar to [the incident with] the estate of a certain lunatic. For a certain lunatic once sold some property. and a pair of witnesses came and declared that he had effected the sale while in a sound state of mind, and another pair came and declared that the sale was effected while he was in a state of lunacy. And R. Ashi said: Put two against two