Can you think of any friendship moments between Dash and Twilight? I notice she had a friendship focused episode with each of the other Manes in Season 1, but Dash didn't get one till s2 (Read It and Weep.)

7299778 I remember them palling around in "Winter Wrap-Up" and Twilight was the only one who seemed concerned for Rainbow's well-being in "Sonic Rainboom."

I don't think there's much of Twilight and Rainbow to actually bond over, given that's Twilight is a bookworm and RD is an athlete, they don't have much to overlay and given RD's and Twilight's characters, they rarely would have the moments to bond.
That being said, there is the hugging scene in episode when the Mane 6 told the CMC about their cutie marks...

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She also nuzzled her in Over A Barrel.

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Okay... I don't remember that scene, but I will for the sake of the argument presume you are correct. That doesn't address my major statement that the two have very little in common with the implication that their bond to each other is more a result of their bonds with the other 4, especially on Twilight's end.