Keith Hernandez hit a two-run homer and added a single to help the Mets to a 6-4 victory over the St. Louis Cardinals and a split of their doubleheader last night in New York. Hernandez, traded to the Mets from the Cardinals last week, thus knocked his old mates out of first place in the National League East, a half-game behind victorious Montreal.

Kevin Hagen, in his first major league start, combined with veteran Bruce Sutter to pitch the Cardinals to a 3-1 victory over the Mets in the opening game.

New York's Walt Terrell, making his first start of the season since being recalled from Tidewater, was credited with the victory in the second game. Terrell pitched 5 2/3 innings, giving up all four runs.

Mookie Wilson led off the New York first in the nightcap on third baseman Ken Oberkfell's error and one out later, Hernandez hit his fourth homer of the year into the right field bullpen at Shea Stadium. It was Hernandez's ninth hit in his last 19 at bats as a Met.

Wilson had a two-run double and Hubie Brooks an RBI single in the second inning as New York boosted its lead to 5-1 off John Stuper (7-4).

Jamie Quirk drove in St. Louis' first run on an RBI grounder in the second and a double in the fourth. David Green hit a two-run homer, his first of the season, to make it 5-4 in the sixth inning.

Rusty Staubknocked in a run with a pinch hit single in the seventh to make it 6-4. It was Staub's seventh consecutive pinch hit, one shy of the major league record by Dave Philley in 1958.

Hagen (2-0), a 23-year-old right-hander, gave up eight hits, walked only one and struck out one in seven innings in the opener. Sutter gave up three hits in two innings as he notched his sixth save.