Yevamot 19A

מבית אחד. יבומי חדא ואיפטור[י] אידך ⁠— לא, דדלמא אין זיקה ככנוסה, והוו להו שתי יבמות הבאות משני בתים. אלמא מספקא ליה!

from a single household, i.e., husband, and everyone agrees that only one wife from each household may be taken in levirate marriage, as the verse states: “To build his brother’s house,” which is interpreted to mean that the remaining brother may perform levirate marriage with only one wife of his late brother and not with two. To take one in levirate marriage and exempt the other without any procedure, he may not do, as perhaps the levirate bond is not substantial enough to make the first brother’s widow like a married woman to the second brother. In that case this woman whose husband died first remains the wife of the first brother, and the second woman is the wife of the second brother. Then there would be two separate levirate obligations, and one could not exempt the other; they would be two yevamot who come from two households. Therefore, apparently even when levirate betrothal was performed R' Shimon is uncertain whether or not the levirate bond is substantial.

וכי תימא: מדאורייתא הכי נמי דמייבמא חדא ומפטרא חדא, ומדרבנן הוא דאסור, גזירה משום שמא יאמרו שתי יבמות הבאות משני בתים ⁠— חדא מייבמא, ואידך מיפטרא בולא כלום,

And if you would say that by Torah law, indeed, one of them may be taken in levirate marriage and thereby exempt the other, and this was prohibited only by rabbinic law, this would be a rabbinic decree due to the concern lest those who were not aware of the details mistakenly say that in general if two yevamot come from two households then one is taken in levirate marriage and the other is exempt without anything. One might have thought that the reason R' Shimon required the other woman to perform ḥalitza is to avoid the possibility of such a mistake.