Both were groomed to be Messiahs but failed, for one reason or another (though being groomed through eugenics is rather different than being groomed through circumstance); both carry a sardonic and world-weary detachment through much of the plot, broken by a few dramatic moments (Fenring's refusal to kill Paul, Quirrellbody's dramatic appearances and disappearances).

That, and Fenring's secret humming language (obviously with a different purpose, but still).