Emailing multiple POIs at the same school?

[MiddleOfSomeCalibrations]

The "Insider's Guide" book says you should not write to multiple POIs in the same program (unless they have "highly overlapping research interests"), because most faculty react negatively to that and it can lead to awkward moments in admissions committee discussions, and that if you write to more than one, "be open about it in your emails." How exactly is that supposed to work though? Would you include that information in the initial email to them? Or would that only be something to include if you have extensive conversations with a POI (like not just "are you taking grad students")?