i dose both tanks... then dose both tanks. within a couple minutes without issue. sometimes i throw it all in together. i figure since i'm going for low maintenance why do it all twice...
besides, if it's fine for hoppy and tom... (thanks a whole lot on that one guys!)
pssst... also, the best root tabs i've ever used (riiz's recipe in the sticky here) has them all rolled up in a ball together to stick in the substrate.
I think this myth arose from mixing solution. We can see iron precipitation by mixing solution. In a large volume dilution, I believe this not to be an issue.