Bjorn, despite a finishing bogey, turned in a second consecutive 5-under-par 66, while Gallacher, whose lone bogey Friday came on the 17th hole, had a 6-under 65. That left them both at 10-under 132 after 36 holes, one stroke ahead of five players tied for third and two strokes ahead of the group of five tied for eighth.
Miguel Angel Jimenez had five birdies in a six-hole stretch late in Friday's round but had a bogey at 18 that dropped him into the group at 9-under. He shot 3-under 68 Friday.
Danny Willett had an eagle and five birdies for the round of the day at 7-under 64. He, along with Victor Dubuisson (65 Friday), Richard Finch (66) and Tommy Fleetwood (68) are tied with Jimenez for third.
First-round leader Anirban Lahiri had a double-bogey 5 at the 15th hole that left him even for the day and among those at 8-under for the tournament.
Bjorn had three birdies on each side Friday and hadn't carded a bogey in the tournament until his 36th hole. He was long with an approach to the 18th green and then missed a 10-foot putt for par.
Gallacher had all seven of his birdies in the first 15 holes before slipping back with a 5 at the par-4 17th.