The kh2po4 and the iron in the csm+b shouldn't be mixed or they can become unavailable for use by the plants. Some people make an all-in-one solution but it doesn't usually include the kh2po4 for that reason. Example is aquariumfertilzer.com sells a macro micro mix and it contains Potassium Nitrate, Potassium Sulfate, Magnesium, csm+b. The problem is you will most likely still need to dose KH2PO4 meaning you will still need 2 solutions (or you could just dose the kh2po4 dry).
As noted in the link posted above, you could try using e300 ascorbic acid in an all-in-one mix to keep the chelators from breaking down, thus keeping the iron complexed and isolated from the po4. However, I'm uncertain about if there would be any reaction once dosed into your tank where the ph would change.