Tech glitch allows you to have T-type and S-type techs at the same time.
This is how it works: (for example)
1. Put Lloyd to the first place of your party and set Genis' Eruption to Lloyd's C-stick shortcut.
2. Switch Lloyd out of your party.
3. Have Genis forgot his Eruption and switch him to S-type. Then battle until he learns Flame Lance.
4. Switch Genis back to T-type.
5. Save the game! (Do not save or look at the tech list after this)
6. Now put Lloyd back to the first place of your party.
7. Go to battle and use that Eruption shortcut with Lloyd. Genis must relearn Explosion during this battle, if not, Eruption shortcut from C-stick is gone.
8. And if you didn't succeed, repeat from the step 5. If you succeed you have now both Genis' Explosion and Flame Lance.