Mixing KH2PO4 and CSM+B in a premix solution, much more concentrated than in the tank, will cause the iron in the CSM+B to form an insoluble precipitate with the PO4, then when you dose the solution you will not be dosing much iron, if any.
But, in the very dilute concentration in the aquarium I don't believe the precipitation is that much of a problem. I dose all of my ferts on the same day, with maybe one minute between the CSM+B and the macro ferts. I don't get any hazy water from doing so. Even if I did, the iron would still settle out on the substrate, where it would eventually be available to the plants. However, I use low light, so lower than usual concentrations of all ferts.