It does sound like you have a 3-way setup. You must replace both switches for a GE smart switch to work. The second switch needs to be an add-on paddle. See the FAQ.
I’ve decided to put this FAQ together to help those with questions about installing GE 3-way switches.
Definitions of terms:
Line: The hot wire from the breaker (usually black)
Neutral: The return wire for circuits (usually white)
Load: The wire that will be hot when the circuit is completed to your light (usually black)
Traveler: In existing 3-way wiring, this wire toggles hot between two switches. In a smart switch 3-way configuration, a smaller charge is looped through the add-on switch.
Some things to know:
3-way wiring for smart switches is different than standard 3 way wiring.
You cannot use dumb 3-way switches with most smart switches (You must purchase the matching GE add-on switch to wire with your GE smart master switch).
If the line is wired into the light fixture, you …