That's not a hard to fix issue, have two reservoirs and one pump.
Pretty much everything but PO4 can be added together, but if the lines come together very close to the tank's dosing point from the pump(eg, this line is very short in length/time in that mixing), this should be okay I'd figure.
The other solution is to make the dosing stock solutions rather dilute.
Like 10X more so. This reduction should be fine, in aquariums, these typically have very little effect once dosed, so if the dilution is high, this should reduce the precipitation.
Not sure, but worth a try.
If you add some HCL, that should help or other strong acids(add these too the water 1st, wait a couple of minutes, then add the ferts to the water/acid mix).
Regards,
Tom Barr
Regards,
Tom Barr