I would say that config-sync has nothing to do with traffic-groups or which device is active.
Preferrably you should configure it on that device, where the new configuration should be active. But at the end it's only important to use the correct "direction" for the config-sync. On the device, where you added the new configuration you have to choose "push" to device-group.
Keep in mind to not change configuration on both devices at the same time to avoid configuration conflicts (the config-sync section will inform you about this).
Hope that helps.
Regards Stefan 🙂