The same two letters, in the same, consecutive order, will change the words below into new ones.

SET
CAP
ART
AD
BASS
WING
CAT

HINT:
The two letters precede some of the words and are inserted into others.

Answer:

The two letters are HE:

SHEET
CHEAP
HEART
HEAD
BASHES
HEWING
CHEAT

Credit: LA Times Crossword
Sylvia Bursztyn and Barry Tunick
HE'S HERE