DRAGON BALL XENOVERSE 2: Cell's & Frieza's classroom meeting

If you want to do this yourself, all you need to do is maximize Mentor meter of those two characters. After You maximize mentor meter of both characters, Do random quest. Parallel quests do fine. After you clear random quest, The game's comment section should inform you that "Something is happening at the Patroller academy." Go straight to the Patroller academy classroom door, and enter. All you have to do then is watch these two individual characters talk it out.
After the scene is over you can talk to them in the classroom. If you exit classroom you can't get this scene happen again with your current custom character.
You also unlock special mark, which includes both characters you just met in the classroom.