Park, who recently missed out on a chance for a fourth consecutive major title at the Women's British Open in Scotland, shot a 66 while Kerr had a 65 at the Royal Mayfair Golf Club.
That put them in a tie for the 36-hole lead at 8-under-par 132.
Ko, still an amateur, became the youngest winner in LPGA Tour history when she captured this event last year at the age of 16.
Park has dominated women's golf this year with six tournament titles, including wins at the Kraft Nabisco Championship, LPGA Championship and U.S. Women's Open.
At the Women's British Open she was trying to become the first golfer -- male or female -- to win four major championships in the same year. She stumbled in that attempt, finishing in a tie for 42nd and needing 78 shots to play the final round at the Old Course in St. Andrews.