I think this time it's more about two units joining forces and fighting together. It shows Sumia and Callum combining; when Sumia is the main attacker Callum boosts her attack and defense, when Callum is the main attacker Sumia boosts his speed, and so on.
It says you can use the system to carry units with low movement, so I'm guessing you might be able to do things like charge in on a pegasus and switch characters to attack.