When the Unison Gauge is full in battle, you will be able to let loose Unison Atacks that allow all four party members to let loose one tech each. If two characters within the sequence use the right teachs, a secret Unison Attack can occur after the basic Unison Attack is complete. The following is a list of all of the game's secret Unison Attacks and the necessary techs and characters that unlock them.
Note that the timing or placement of the tech inputs are not important. So long as the right techs appear within the Unison Attack sequence, the secret Unison Attack will occur afterwards regardless. Also note that even though every secret Unison Attack has more than one tech combination for unlocking it, this table only lists one tech combination for each of them. Regardless of which techs you use to unlock the secret Unison Attacks, you only get one version of each secret Unison Attack anyway.