The system of combo mode is a skill which you gain after at lvl10 (Cabal Alz) and finishing the requirement in Cabal. The combo mode need at least 3-5 skills so that to do properly. About LVL 15+ you will be able to do the combo mode successfully a couple of times not just after two skills. The combo mode does take some practice to get use to and to control.Its not easy at first, trust me its not, so do not get mad if your not good at in game (cheap Cabal Alz).
All in all, you will see a bar on your screen once you select the skill of combo mode. The bar has a meter which fills to three colours: Green, Yellow,