So let's say you are specced into defile and have a full charge bar, you cast defile on two targets and look at both, you will notice that instead of your charge bar draining twice as fast it will just remove defile off of your first target. Nowhere on the tooltip does it say you can only have defile cast on one target.

Also if you have two locks specced into Torture, and both cast it one will be overwritten which wouldnt be so bad if the end proc wasn't damage based, instead one of you loses the damage.