How to do that?
This is how it was:
--- a few thousand years ago
Vulcans (savage and aggressive)
--- not clear ---
Vulcans find logic and control feelings - some leave Vulcan and get to Romulus
(not sure what was first)
--- a few thousand years less ago

---
Vulcan: Vulcans developed, logic, suppressed feelings
Romulus: Romulans developed out of Vulcans, feelings, aggressivity
They didn't mix because they became what they are on different planets.
A Vulcan that would not undergo all that training would not be like the Vulcans are today. There was a Voyager episode that showed it. It showed what Tuvok was like in his youth and it showed the rituals needed to make a "true" Vulcan out of him.
Have this in mind: A Klingon without training can't fight either. So you're not born a warrior, you become one. You are not born as a logical being, you have to become one. Get it?