I think there's two reasons why you don't want to mix macros and micros in water.
1. The EI method of dosing calls for macros and micros to be dosed on alternating days, not at the same time. And there's a reason this method exists, because it works.
2. If I remember correctly, there is a conflict between the two that will cause one fert to precipitate out of solution. If it precipitates out of solution, it isn't getting added to the aquarium.