Siman 412

One who erred in the law, thinking that he could establish an eruv for half the day in this direction and half the day in the other direction, and placed two eruvin, one to the north for the first half of the day and one to the south for the second half of the day, or if he told two people, "establish an eruv for me," and one established an eruv for him to the north and one established an eruv for him to the south, and each one placed his eruv at the end of two thousand cubits—behold, this person shall not move from his place, because it is not known which eruv was acquired for him. That is to say, it is certain that he himself, when it is revealed to him that he erred, will say that if so, his intention was only that one of them should be acquired and not the second. But this itself is not known, neither to him nor to us, which one is the one that should be acquired and which should not be acquired; therefore, of necessity, he shall not move from his place, and he has only four cubits, which are necessarily given to every man.

And if the two eruvin that he placed, or his two agents who placed them, were not at the end of the techum—he walks whatever is possible for him by virtue of the two eruvin. How so? If each eruv is at the end of one thousand cubits from his house—he has one thousand cubits from his house toward each of the eruvin. And if one eruv is to the east at the end of one thousand cubits, and the second is to the west at the end of five hundred cubits—he walks one thousand cubits to the west by virtue of what the eruv in the east allowed him, and to the east he walks one thousand and five hundred cubits by virtue of what the eruv in the west allowed him, and so too anything similar to this. But we do not say that the two eruvin are nullified and he has only four cubits.