Any time you backstab you have a chance to backstab twice. Each attack has a separate roll to see if it hit's or misses. It also has a separate roll for damage.
I've never seen a double 500 BS on anything but a sitting player in a duel. But double 400s can happen with raid buffs and a high ATK stat (wolf form, bard epic, etc)
See
http://wiki.project1999.com/Mechanic...e_Calculations for a little more information on Backstabs in general.