If you want, you can set an association in the opposite direction, with the master as the trigger and the accessory is the target. I think that will work, although I don’t know 100% for sure with this specific models. You can certainly try it and see. The only question is whether you’re going to get a loop, but I don’t think so.
If that doesn’t work, you can use the “mirror“ function in the official smartlighting feature in smartthings and have the accessory mirror the master.
FAQ: create a virtual three-way using the “mirror“ feature of SmartLighting [NEEDS UPDATING]
As far as having them both look like one switch, they are not one switch and they will not look like one switch. They each have a radio, they each have a device ID, they each have routing, and they are not one switch.
You could create a virtual switch and use webcore to tie everything together, but that would be a lot of work and then you would end up with three devices in your device list, instead of two. So I’m not sure if that’s what she would want.