JOAT's link to the PWW box joint jig led me to dig up this article by
Lorne Gushulak. He's discovered an alternative method of cutting box
joints by ganging the sides together and... well it's hard to explain.
The link has been posted here before, but it was about 5 years ago, and
so I hope someone finds it useful.

http://www.woodworking.org/WC/GArchi...hboxjoint.html