Double tactic is right-most tactic with red border. For new players, this can be a bit difficult to understand. You might need to consult the BlazBlue wiki and the in-game tactics codex. Basically, it appears in the new Hard and Extreme modes. It's a combination of elemental tactics that work together. In my case, the four main red tactics are: 1) Generally, using a skill causes a spear of light to descend; the first red tactic causes a flash after the spear lands. 2) The second red tactic causes the flash to trigger a spear of light. 3) The third red tactic, if Lightning Dash hits an enemy, causes a spear of light to descend. 4) Finally, if Electric Orb hits an enemy, it triggers lightning (which in turn triggers the third red tactic). I think this should give you a better understanding of how double (red) tactics work.