Quest in Kakarot, then F6 into a real Sparking! ZERO arena duel from your own install.
Play DRAGON BALL Z: KAKAROT as usual. When you want a proper arena scrap, press F6 (or F7/F8 if F6 is taken) and DRAGON BALL: Sparking! ZERO opens from your install for a real duel. Close Sparking! ZERO and you're back in the adventure.
Solo. Melty starts Kakarot with a small Arena Gate panel. Your Sparking! ZERO fighters and stages stay in your game folder — nothing is copied out.
Hotkey duel gate + return to Kakarot. Story-marked foes come later.