Feng shot a two-under-par 70 to lead the field on eight-under-par at Trump National Golf Club in New Jersey.
Choi, 17, scored the joint lowest round of the day to finish six-under-par at the halfway stage.
Korea's Jeongeun6 Lee and Amy Yang are also tied for second on six under.
Lee is playing in her first US tournament and the '6' in her name distinguishes her from five other Korean LPGA players.
England's Charley Hull made the cut and is tied for 14th on two-under-par after a two-under 70.
Scotland's Catriona Matthew also made the cut and is on one under after her second-round 71, one shot in front of England's Georgia Hall.
England's Jodi Ewart Shadoff missed the cut with a five-over 78, leaving her at five-over.
The day began with 39 players finishing their first rounds, following weather delays on Thursday.
Former champion Michelle Wie withdrew from the event with a neck injury, after just one hole of her second round.
Teenage Korean amateur Choi Hye-Jin carded a second-round three-under-par 69 to sit in a tie for second at the US Women's Open, two behind leader Shanshan Feng.