Using chess as analogy only makes sense when both players normally alternate turns, despite the difference in speed (if any). When "how often you get a turn" is quantified in numbers, you have to refer to turn gauges instead, which basically is a bar that fills up according how fast your character is, and when it's full, it's that character's turn. When it's 25 vs 10 agility, for example, the character with 25 agi will have their bar filled up two times and a half before the character with 10 agi finally gets a turn. which is why if your agility is 0, you'll never get a turn.
I've never known a game that has two types of speed like this though, cause it's simply redundant. Going by this manga's terms, a character with double the speed of their opponent will attack twice as many times. A character with double the quickness will also attack twice as many times. Not just attack, includes using items or spells and stuff.
This is why mixing turn based elements into real time scenario makes no sense, since despite it being someone else's turn, you can still act.