Finch capped a 5-under-par 67 with his birdie at the 18th hole for a 36-hole total of 8-under 136. Foster, who had a string of four consecutive birdies, turned a second straight 4-under 68 for his share of first halfway through the tournament.
They are one stroke ahead of first-round co-leader Brett Rumford, who had a 70 in the second round, and Paul Lawrie (69). Gregory Bourdy, who tied for the round of the day with a 6-under 66, and Paul Waring (67) are in fifth at 6-under 138.
Knut Borsheim, who was tied with Rumford for first after Thursday's play, was 1 over for the second round and slipped to 4-under for the tournament.
Finch opened Friday's play with a 3-putt bogey at the first hole. However, he worked to 3-under for the day before he had a bogey at the 13th hole for the second straight round. He answered with birdies at 14, 16 and 18
Forster played the second nine of the Gleneagles Hotel course first. He was even -- with a bogey at 13 and a birdie at 14 -- through eight holes. He dropped in a birdie at No. 18, starting a run of four straight birdies.