Characters can combine magic to create new spells.
Before a combination can be initiated, both members must be to at least Banalet class (or you must be Aqutallion to use Devastate).
Then have both members select their spells (the member using an elemental sword should just select "Fight"), then at the last prompt before a battle round begins, choose "Combi", and select the two members, then OK.
(Again, the two members must be at least Banalet. Dan can't combine magic, and neither can Kustera.)