When I was in high school, you carried your backpack slung over one shoulder. Always, without fail, on pain of being the most complete loser in the whole school. I had one acquaintance who was on crutches for about a month, and she one-strapped the whole time, even though the pack fell off her shoulder about every third step.
So I was bemused to discover that one-strapping is now completely out, and cool high school kids are using both straps again. Forrest Wickman of Slate conducted a small investigation, interviewing 75 acquaintances, and he found that everyone from the class of 1994 or earlier one-strapped. Between 1995 and 2005 there was a mix, but one-strapping still dominated. After 2005 things switched over and two-strapping became the majority position. By the class of 2008, two-strapping was universal.
So that's at least one way young people today are more sensible than their parents were.