I'm just coming up to speed with EI dosing and have never actually started a planted tank, so please take this with a grain of salt:
My understanding is that if you dose the micros and macros at the same time, you risk forming ferric phosphate, which is relatively insoluble in water. Therefore, you won't get the iron into the water column where the plants have access to it.
I think that there are some forms of chelated iron that will not form ferric phosphate, and some people dose both macro and micro on the same day, but usually not at the same time. I think that if you mix the micro powder with the potassium phosphate powder, you are almost guaranteed to precipitate ferric phosphate.
I suspect there is a way to accomplish what you want to do, but probably not without a secondary iron additive.
Hopefully you will get some responses from folks more experienced than me.