And really, Romulans are just an offshoot of Vulcans already, so if they could split once, they could do it again.
That's already been to done to some degree with the Romulans since the Debrune were described as "an ancient offshoot of the Romulans", but also at the time of the Vulcan-Romulan schism.
As for the split of the Romulan Empire being split into two entities, that didn't necessarily follow that civil war would erupt in the near future with one side achieving total victory over the other. Outside of the Korean War, the north and south have remained at peace for what's now almost sixty years. Of course, with the RSE being part of the Typhon Pact and the IRS possibly being part of a greater Khitomer Alliance, that creates a very tense powder keg.