Cumpton (1-1) allowed one walk and struck out five in seven scoreless innings to give the Pirates a sweep of the twin bill.
The Pirates, who won the first game 2-1 in 11 innings, extended their lead in the NL Central to 1 1/2 games.
Tyler Lyons (2-3) took the loss for the Cardinals, who lost their season-high sixth consecutive game. Lyons allowed four runs -- three earned -- on eight hits over six innings.
Notable deaths of 2014 [PHOTOS]
Julius Randle breaks leg during NBA debut