He cared as little about sport as Magnus did about poetry.

This probably was the reason the two were such chums.
They never trod on one another's toes.
When they went for a walk, Joe usually dawdled along trying to think of rhymes for "nightingale," and "poppy," and "windmill," and the other beauties of Nature which met his eye or ear; while Magnus stopped behind to vault gates (which always caught his foot as he went over), and do "sprints" with wayside animals, in which the wayside animals usually managed to pull off the event.

I'm not sure that they ever talked to one another, which again may have been a reason for their great friendship.

If they did, nobody ever heard them; indeed, they never seemed to look at one another, or to be aware of one another's existence, which no doubt fully explains their mutual devotion.
The only real bond of sympathy that I can think of was that they were always going in for examinations together, and always getting plucked.
Had the name of either ever appeared on a prize list, I am convinced there would have been a panic in the school.

Even when they entered for the Wheeler Exhibition for boys under 15, Joe being on the day of examination 14 years 364 days, and Magnus being a week younger, no one supposed for a moment they had a chance against the fellows of eleven and twelve who went up against them; and no one was disappointed.
I asked Magnus afterwards how it was he came to grief.
"It was those beasts, the Greek gods.