I've been dosing the stuff at the same time and also one day apart just for testing. There is no difference really. Dose PO4 and Fe where you have good current such that it will be carried away immediately. That's all.

Don't mix PO4 and Fe in one bottle though. It will precipitate.

What's going to happen if a new Fe dose hits residual PO4 remaines for the more chemical challenged.

Best regards,
Detlef