You have to watch the buoyancy characteristics of tanks that come from different manufacturers. Luxfer and Catalina aluminum tanks can have VERY different buoyancy characteristics. The Luxfer tank will be +4 when empty, but the Catalina may be neutral; if you try to double them up, you'll end up badly out of balance. It's best to double up tanks from the same manufacturer and from the same series (for example, Faber has tanks that are the same size but VERY different in characteristics).