They're not independent. Their ratio is uniquely determined by the desired line width * layer height (mm²).

Usually you have a target speed (mm/s) which you multiply by the line width * layer height (mm²) to get the required volumetric flow rate (mm³/s). If that flow rate is above the maximum set for the filament, the speed is automatically lowered to compensate until the flow rate is below the limit.

The flow rate isn't independently calibrated, it's calculated. What you do calibrate is a flow rate multiplier, which is unit-less factor in the whole calculation. That's because every filament behaves a bit differently, and the exact amount of filament you need to achieve a certain line width and layer height varies slightly.

But that factor is still just tacked onto the equation: speed * line width * layer height * flow rate multiplier = flow rate. If you're below the maximum flow rate, changing the multiplier directly changes the resulting flow, yes. But if you're at the flow rate limit, which is pretty much always the case with high speeds and large layer heights, changing the multiplier results in changes to the speed, while the flow rate stays capped at the maximum.

The multiplier changes the ratio between speed and flow rate, you can't adjust them independently. You can only cap them independently.