This switch is designed so that it replaces one 2-way switch. The second 2-way switch is either rewired as a SPST switch or can be replaced with a regular SPST light switch (a not two-way switch). This Lutron switch controls the light. The second switch only signals this Lutron switch when the second switch changes state (flipped up or down).
https://www.amazon.com/Lutron-MS-OPS5M-WH-Single-Pole-Multi-Location-MS-OPS5MH-WH/dp/B005WM3C42