Watchtower is adapt at stopping schisms before they start. Any groups have have broken away have been very small...the largest no bigger than a circuit. The cult mechanism is in place to quarantine any dissenting individuals and demonizing them to the max.
A schism would need to consolidate around a leader or leaders from the top. Of course, in seeing what happened to Ray Franz and Ed Dunlap, Watchtower will go into witchhunt mode and kick out any dissenting leader in the street. I believe had Ray Franz chose to do so, he could've started a competitive schism. But he was a man of integrity and did not want the mantle of leadership and the harm that can bring those under that mantle.
Even if there was an even split in the Governing Body to the breaking point, I wonder how Watchtower would handle it. Eventually one faction would win out and shut and excise the other down.