Board Thread:LOC General Discussion/@comment-5.146.244.188-20130826070957/@comment-5616072-20130826194943

Dual skill cards contribute their full potential on the line that they're on and 20% of their potential to the other line.

SO... If you have a card that causes great boost to ATK/DEF (and it is Skill 10) and you have it on your ATK line then it would boost your ATK 20% and it would also boost your DEF 4% (20% of 20%.)

Originally the Colosseo information stated that these dual effect cards would have a 50% effect on the "other line" but they revised that later and stated that it was actually only 20%.