As the title suggests what is the nature of the Vulcan Romulan divide?
Is it biological, are the Romulans biologically distinct enough to be considered a different species ?
For example would a medic treating injuries in war time need to check if the patient was from Romulus or Vulcan in order to provide adequate care ?
Or is it political with the only thing separating them is their differing philosophies ?
Initially a political issue which drove the two groups apart (whether to adopt the teachings of Surak, based in logic and reason) or to continue the more warlike nature of the Vulcan species. Those who rejected Surak would leave their world and become the Romulans.
When Surak's reforms of embracing logical principles and rejecting emotions spread rapidly across Vulcan in the 4th century, a minority rejected Surak's ideals. Those who marched beneath the banner of the raptor, which became the symbol of the Romulan Star Empire, departed Vulcan in the 4th century.
Later, some of their descendants established settlements on the planets Calder II, Dessica II, Draken IV, Yadalla Prime, and Barradas III. An ancient offshoot civilization, called the Debrune, at one time existed on Barradas III, but it had died out by the 24th century. (ENT: "Kir'Shara"; TNG: "Gambit, Part I")
Being Romulan is as much a political identity as a physiological identity. The Romulans also lack the rigid mental disciplines common to their Vulcan cousins. These mental conditionings can affect the Vulcan physiology, particularly their brains in a number of ways.
2,000+ years of differentiation between the two groups, as well as living on different planets and exposure to different ecosystems has lead to the two groups having similar overall biological capacities but distinctions due to their time apart.
Both groups are still able to reproduce with each other without technological support.
Yes, there are some differences between the two groups that would require a medic to take their species variation into consideration when treating them. A lack of knowledge about the Vulcan healing trance for example might be dangerous to a medic skilled in Romulan physiology but lacking in training for Vulcan physiology.
Vulcans learned to gain conscious control of many of these functions, allowing them to regulate their bodies to a high degree by simple will power. When injured a Vulcan could go into a trance-like state, using this ability to concentrate all of his or her energy onto repairing the injury. (TOS: "A Private Little War")
This trance could be self-induced and gave the physical appearance of near-death. It was similar, in principle, to Vulcan neuropressure techniques which could be used to relax the mind and body. (TOS: "By Any Other Name"; VOY: "Riddles"; ENT: "The Xindi")
