For those not using a dosing pump, but otherwise automated dosing thingamajig...

[Delphinus]

For the most part in a prebalanced tank, you will add both at fairly close the same rate. You could always do a manual adjustment say once a week if there was drift in one of the parameters.

Having said that, I would prefer being able to add both independent of one another.

I know with the Masterflex peristaltic pumps (lab grade, very expensive, although the occasional good deal comes up on ebay .. also LittleSilverMax had at one point, one for sale, if anyone wanted one of these they should send him a PM) can have interchangeable heads to allow for different flowrates which basically involve different sized tubes or different diameters of the rollers). But the speed at which the rollers turn would be the same for each head so it's still not AS independently adjustable from each other that two seperate feed pumps would be.