If you want to have a second tube adapter for the same metering valve, not only will you need the ferrules buy also a nut. Once you screw down the nut, it makes the ferrules bite onto the tube and makes a permanent connection. I would just bring the metering valve in with you and have them set you up. That way they will make sure you get what you need. If not, just tell them you need a set of front and back 1/8" tube brass ferrules and a 1/8" tube nut.