The user manual says nothing about setting up two email addresses. It would seem to be impossible. The app automatically choses the email address in my profile and doesn't seem to allow for a second email notification. At least that's how it seems to work. But the manual says nothing about it.
Also, the user manual was almost impossible to find. It's not included in the box, there's no mention of it anywhere (the paperwork included only refers to installing the app), and I can't even find a page or reference to the S160 Water Sensor anywhere on the Dlink site. I finally goggled "DCH-S160 manual" and found a pdf.