because of cadence discharge “offhand” fast charging cadence
cadence itself only hits with 1h on the actual cadence hit, regardless of dual wield or not
what it then does is default to regular dual wield interaction, which is 50% chance dw hit 25%chance lefthand hit 25%chance right hand hit
so on those 50% occurence where you hit with both hands on the cadence discharge hit, but only 1 of those hands is the actual cadence hit, the other hand serves as a regular hit, thus grants a cadence charge.
Since this occur on the actual cadence hit it means you’re immediately getting an instant cadence charge on the discharge itself, so now you only need 1 more regular hit for the final chargeup.
some more reading if you’re interested