From what I have learned so far, the only place where it is acceptable for anyone to wear another's letters is spouses/engaged couples. I wear my husband's letters, and, on formal occasions, his badge--and as his wife, I am the only one who can.
If I am initiated, he would most likely wear a lavalier from my GLO w/ his alumni recognition pin. I then would wear his lavalier w/ my own badge.
The way I understand it is: wearing anothers' badge, letters, etc, is the equivalent of a wedding ring--with more details about the affiliations and principles of your spouse.
Conversely, there is no way in hell you will catch me wearing my SILs badges. I would catch an earful if I even suggested the idea!
Adrienne (PNAM-2003)