But couldn't they have chosen it?
: During the battle for Sorrow's End Dart calls Shushen "little soul-brother" though. To me that always indicated that they were the same as Cutter and Skywise.
In fact I always got the impression that Cutter and Skywise chose the brother-in-all-but-blood thing, it was just the soulname exchange that wasn't quite voluntary.
Hmm, the "soul-brother" titutlation has only been used in these two cases as far as I remember and to me that always meant that, as you say, the soulname exchange wasn't voluntary.
I think Skywise and Cutter (and by proxy, Dart and Shushen) are that close BECAUSE they're soulbrothers, even before they Recognized. So the matter of choice is a bit blurred 'cause their personalities and the way they grew up meant that they couldn't really have chosen otherwise.
We really don't have enough to go by to determine the exact rules IMHO, it all depends on how you read it I guess.
That being said, I would love to see the same scenario with two female elves. Close friends/lovers who are so intertwined at the most basic level that they essentially Recognize.