I’m calling the primary switch the one connected to the load. Associate the secondary to the primary with group two. Do not modify association behavior on either device. This will allow you to turn the load on/off from either switch. With just the association setup the secondary switch did not indicate the proper state and their states in ST app were not correct. I then setup rules to mimick states using smart lighting. Ie if primary turns on/off then turn on/off secondary and vice versa.
This will probably work for other hubs but I’ve tagged it smartthings for now.
You’re missing a couple steps. If you want the state to be updated on the Secondary switch then you also need to associate the primary to the secondary.
Then, since you’ll have two-way associations, you WILL want to change the association behavior on the secondary switch or else there will be an endless loop (primary updates secondary which then updates primary…). To prevent that, set the Associate Behavior to 11 on the secondary swtich (leave the primary association behavior at the default 15)
After that, physical button pushes on either switch will update the state of the other switch and you won’t need special rules to keep them in sync. All other automations/rules, etc should only send commands to the primary switch, which will then update the secondary switch state via association