Yeeouch. Not good.
I just got a tandem (beginner's tank bike, 1979 Schwinn Twinn Sport 10sp), and will have to watch for this. Hopefully I can gain from your (or your stoker's) loss. Please give her my sympathies - I hope those cuts and bruises don't bother her for long.
I guess the good news is, you didn't try to adjust your gloves while doing 30 down that hill.
Recall the t-shirt supposedly worn by some motorcycle rider whose ride had a long seat for a rear-mounted companion? The shirt said on the back, "If you can read this, the b____ fell off!"
Perhaps we can make some like that, with the word "stoker" substituted where appropriate. And with a picture of a tandem on the front, of course, for the uninitiated.