Legaia 2: Duel Saga

In order to use variable arts you need to be taught how to use them through story line. Each character has two variable arts. To use them both characters need to attack the same person. You cant add any extra commands after entering the art. And yes your friend was correct, one starts the attacking combo and the other finishes it.

Once you have unlocked the first 2 variable arts in the storyline, you can use any variable arts. To use variable arts, both characters need to have 80ap. One character has to use their respective variable art A, the second needs to use their variable art B
For example, Lang uses Solid Strike <><v^v which is variable A, and Maya uses Shockwave <>v^<> which is variable B.
You can use any combination of characters, as long as one uses variable A and the second uses variable B