Cordial colleagues is about right, and certainly very good friends though
not ideological twins.  The Twain-Howells correspondence has been edited
by Henry Nash Smith and William Gibson and published in both a 2-volume
and a 1-volume edition.  (I'm still in the market for the 2-volume opus,
if anyone has a set to unload.)  The folklore is that Howells bowdlerized
Twain, but the more accurate view is that Howells, through the _Atlantic_,
legitimized Twain by making him more than a "mere" humorist.  See Edwin
Cady's 2-volume biography of Howells (_The Road to Realism- and _The
Realist at War_).

Peg Wherry (your resident Howellsian)