What does your batch contain? How are the machines connected to one another?
For example, if you compressing just one small file, there is probably no benefit to be gained from using multiple computers - the time to split the job and send media back and forth will negate the advantage of extra processing cores.
The bigger the tasks, and especially if there are multiple tasks in a batch, the more it is advantageous to enlist the efforts of multiple computers.
Also, as you probably know: besides using a "render farm", there are benefits to be gained by allowing "extra Compressor instances", even in the same computer. Since you are using 12-core machines this can be quite relevant. Preferences->Advanced:
"Enable additional Compressor instances".