Use magic successively
Double is an indirect magic spell in Final Fantasy VIII. The spell gives characters the ability to cast two spells in a single turn. It can also be junctioned to boost stats. Double is superceded by Triple, which gives characters the abilities to cast three spells in one turn.
Double is a common spell later on, but early on can only be drawn from a select few enemies and bosses. It can be refined from Dragon Fins using the Guardian Force Diablos' ability Time Mag-RF. This makes Grendel a strong source of Double, as Dragon Fins can be obtained from the enemy and Double can be drawn from them.
Double inflicts the Double status effect which grants a character the ability to cast two spells in one turn for the duration of the full battle. It can be cast using the Magic command or Selphie's Slot Limit Break. Casting Double in battle increases compatibility with Carbuncle by 1 and with Cerberus by 0.8, but lowers compatibility with Eden by 0.2.
The support ability Expendx2, granted by the Guardian Forces Cerberus, Bahamut, and Cactuar, increases Double's potency, as it means that spells cast twice with Double use up only a single spell slot.
Double's junctioning to stats is relatively low, with the exception of Hit-J. Therefore, Double is best used when casting spells.