I vex my heart with fancies dim: He still outstript me in the race; It was but unity of place That made me dream I rank'd with him.
And so may Place retain us still, And he the much-beloved again, A lord of large experience, train To riper growth the mind and will:
And what delights can equal those That stir the spirit's inner deeps, When one that loves but knows not, reaps A truth from one that loves and knows?
But, Tennyson admonishes himself, he's just making a big deal out of nothing.
Arthur "outstript [him] in the race," probably meaning he was of higher social station and more intelligent and accomplished than Tennyson. This could also be a deliberate posture, though, to make the writer look more modest than he actually is.
But even though Arthur was better (or so Tennyson imagines), they appeared to be similar because they were in the same place at the same time (like the dramatic unity of place).
Tennyson's hoping that, once they are together again (presumably in Heaven or whatever immortal realm exists after death), his friend will be able to share that experience with him to make him better. Because there's nothing better than the feeling you get from learning something from someone you love.