It IS possible for the same monster to be mugged simultaneously by two different players. A player may miss a secondary mugging because the monster has run out of gil, however.
Success rate is NOT 100%, though is very high.
Usage of this ability in Ballista will allow the user to steal petras.
The amount of gil you can mug depends on the amount of gil the monster can drop. However, mugging an enemy does NOT decrease the amount of gil dropped.
Assassin's Bonnet causes the THF to mug a larger percentage of a monster's gil than usual.