1 or 2 Aeon Labs DSC18103-ZWUS for 3-way switch setup

3-way

#1

I bought two Aeon Labs DSC18103-ZWUS for a project I want to tackle. I have a 3-way set of switches that control some backyard flood lamps. One switch is downstairs the other is upstairs. I bought two thinking I need one at each of the switches however, from what I am reading it sounds like only one maybe needed. I also have a GE12722 handy if needed.
Any info would be appreciated like if only one micro switch is required should it go into the box with the main power feed to it or the secondary switch in the line. Iv’e not checked but imagine the primary is the downstairs switch and the second is upstairs although the lights are right outside the second floor switch