"As I pondered weak and weary, upon a midnight dreary, over many a volume of quaint and forgotten lore"

I ran across Knot for Climbers, by Craig Luebben,(1993) a very decent reference. The square knot, backed up with a double fisherman's knot, is his preferred choice for joining two rappel ropes together(!, over a double fisherman's and a figure eight. Luebben is an excellent, experienced rock climber and his choices have worked well for him. So you guys are not going to die after all!

Just be careful out there (with any knot). I still have reservations about the square knot - it is real easy to tie improperly, especially if you are a bit tired. Luebben does repeat the information that a rope tied with a square knot retains 45% of its strength.