Suppose that Edward is married to Susan and Michael is married to Susan's sister Judith. Edward is therefore Judith's brother-in-law, and Michael is Susan's brother-in-law. In my usage, and what I think is standard English usage, there is no named relationship between Edward and Michael. In particular, they are not brothers-in-law. I was therefore surprised to see a news item in which men in this situation (one of whom is accused of trying to hire an assassin to kill the other) were described as brothers-in-law.
There are languages in which the relationship between Edward and Michael has a name. In Carrier, this is the -loh relationship. One could say Lhloh 'uhint'oh "they are each other's spouse's sibling's spouse/sibling's spouse's sibling". (For extra credit, try to pronounce the onset cluster [ɬl].) German Schwippschwager seems to mean the same thing. The term "co-brother-in-law" is apparently used by some authors as a translation of such terms, but doesn't seem to be in natural use.
What I'm wondering is whether the news item that described Edward and Michael as brothers-in-law is simply in error or whether there are native English speakers for whom this is correct usage.