Combining active time battle and unison skills is not easy to implement in terms of both the battle system and the game play. But we really wanted to make it work, because it makes the battles much more intense and interesting. When you are being attacked by strong enemies, are you going to fight back, heal, or are you willing to wait for your unison partner's action barto be filled up so you can deliver a powerful blow together? Is it worth it to wait? Is the cost of both of your turns worth the damage you'll deliver? You have to access the situation both you and your enemies are in and make a decision in seconds.
In Vanished Souls, the power of unison skills are based on each character's strength and the nature of each skill. For mages, it's most likely their MAT. Julian is fast. It's often based on his AGI, unless the skill doesn't require speed input. Be aware that if your enemy is resistant to the weapon you use or the element of the skill (earth striike, for instance), your attacks are not going to be effective. So unison skills aren't always the answer.
Let's look at how Soren was trying to use a unison skill. He did have enough skill cost. But Julian was not ready.
Try it out. Inexperienced players can start with a easier mode and get some battle trainng first (And, no, you can't switch fighting mode during the game). You'll love it. It's addicting.