I have never heard "a pair of ruler" but I wouldn't be surprised if someone on this planet said it that way.
SingularI have a ruler.
Here, borrow my ruler.
Thank-you for lending me your ruler.
PluralTeacher, I can lend John one of my three rulers.
John now has one ruler and I still have two rulers.
When John returns the ruler he borrowed from me, I will again have three rulers.
Others thought it was odd that I kept three rulers in my desk when most students only have one ruler.
Ruler as a personA ruler is the head of a country. 9singular)
Historians have identified the early rulers of most nations. (plural)
While some people may say "pair of rulers" when they mean "two rulers of two different countries", it is better to use a number, such as "Countries could experience conflict if they had two rulers at the same time."