Easier later in the night or in the epilogue when the children are more active, but possible as soon as Tim and Tom are both active. Go on and off the bridge, end to end, until you see a twin spawn at the end but not chase you. Turn around and you'll see the other one. Keep looking from one to the other until they get close, then fall off the bridge. You'll likely die after this, but as long as you're off the bridge you'll get the achievement.