Post by rapscallion
Yes. Both weapons can proc, and you will have the effect on you twice. If the same weapon procs twice, it will just refresh the buff instead of stack.
The weapon proc from Mongoose works off of a procs per minute formula. Basically the game takes your weapon speed into account when determining the % chance mongoose will go off each hit. Assuming only white damage, mongoose will average out to 1 proc per minute.
The formula does not take into consideration special attacks. So with all of the special attacks, you will see that it actually procs more than once per minute. This makes dual mongoose more intriguing to mutilate rogues since your main special attack uses both weapons.
Also, you will find that the slower the weapon you use, the more often it will proc, not that it really matters for rogues.
You can see a better explanation, and some examples on how PPM works here:
http://www.wowwiki.com/Procs_per_minute