Why did General Erinmore quote a Rudyard Kipling line here?

Sure, he's saying that one [or two] can travel faster than a squad.

But why quote Kipling?

It's in character for a military officer of the time. Classical education.
This type of quote, off the cuff being able to fit a literary quote to a given situation is a kind of 'verbal armoury' of sorts. This type of education and upbringing would arm such a person with a lot of similar quotes, one for each situation. Some people in this day and age can do the same with pre-learned jokes, to be fired off without needing to come up with an original witticism.

I recall a similar one - though I cannot recall the source - from a naval story. In the British navy a seaman must ask a senior officer's permission to grow a beard. He then has two weeks trial period before showing the result to the officer to obtain the necessary permission to continue.

The officer writes just "Matthew 26:41" on a slip of paper & hands it back to the seaman.
On checking his bible* he discovers the passage - "The spirit is willing, but the flesh is weak."

*One must assume everyone in those days would have a bible and be aware that's what the reference indicated.

Because it fits the situation. Two people can travel faster than a pack of people, who must wait for all in their pack to reach specific safe points. While more people would be advantageous if they encounter enemies, Schofield and Blake are attempting to go undetected to deliver a message, and so are less apt to be detected due to noise or sight lines than a larger group.

According to the instructions received, the two soldiers must have seen clearly, that they are going as a pair. They wanted to know, if there are another teams, that could be successful, if they fail.
Risking 1600 lives, there certainly wouldn't be wise to depend on only one way to deliver the orders, but from many reasons General couldn't reveal it.
In order to avoid outright lie, he quoted fresh Nobel Prize Laureate with meaning something like:
You are going to Hell or to Heaven and you are alone in this quest.