If possible, I would separate those two in a dog crate with food and water, spray their bottoms with BluKote, and spend some time with the flock to find out who is doing the picking. Many people feel that feather and vent pecking may be do to some sort of deficiency that they have. Some give their chickens a little meat or tuna in addition to their feed. This increases the protein somewhat, but chickens are omnivores. Most feeds used to contain meat byproduct meal, but most have removed them in recent years. I am a label reader, and occasionally will find a regional brand that still has some. Letting them get out to free range, if only for a few hours a day will allow them to find minerals, bugs, and grasses they require. I hope you can stop this from becoming a habit they all develop.