ב"ה, י"ד תמוז, תשי"ב
ברוקלין.
שלום וברכה!
... כיון שכותב במכתבו שזהו ענין דשני אחים ושתי אחיות, הנה מאן דלא קפיד לא קפדינן עמי', והי' מהראוי אם באפשרי הוא שידורו לאחרי חתונתם בעיירות שונות...
B"H, 14 Tammuz, 5712.
Brooklyn.
Shalom u'Vrachah!
Since you write in your letter that this is a case of two brothers and two sisters, behold: "For one who is not particular, we are not particular with him," and it would be proper—if possible—for them to live in different towns after their weddings.
Summary
The Rebbe writes that when two brothers marry two sisters, there is generally no need for concern unless one is particular about such customs. Still, if possible, it is preferable for the couples to reside in separate towns after marriage.