Petersen completed two rounds at the Bulle Rock course at 8-under 136. Webb, despite suffering a double bogey, was at 137 after shooting 69 Friday.
First-round leader Birdie Kim and Brittany Lincicome shared third place at 138 in the second major championship of the year on the LPGA Tour. Annika Sorenstam was at 139 along with Paula Creamer, Morgan Pressel and Meena Lee.
Michelle Wie, involved in a controversy surrounding her withdrawal from last week's tournament, barely made the cut at 3-over 147.
Wie was on her way to firing a round in the high 80s last week when she quit, citing lingering wrist problems. She later showed up to practice for this week's tournament, causing Sorenstam to say Wie lacked "respect and class."