I installed an Eaton RF640 dimmer and RF642 companion dimmer. I created an association in Smartthings which works, but only the 640 works when using the actual switch. How do I pair/associate the 640 and 642? Do I do it through Smartthings or to each other?
Those don’t match the model numbers that I am familiar with. 
By any chance did you mean the RF9640 and the RF9642? Eaton does make a device with a model number of 640, but it’s not RF and it’s a filter unit, not a light switch. (First rule of home automation: “the model number matters.“ The exact model number. 
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I’m not up-to-date on the various methods of doing this within smartthings (smartthings app is no longer voice navigable), but (assuming I have the model numbers right) essentially you want to use the 9642 as the trigger (the “if“ in an if/then rule) and the 9640 as the target (the “that“ in an if/then rule).
If you are using zwave direct association, then you would store the device ID of the 9640 in the association group list of the 9642, that is, you tell the trigger device what targets it is allowed to send commands to. Also, you have to be using association group 3 for dimming, not association groups one or two for that particular model.
https://products.z-wavealliance.org/products/3157/assoc?noFilename=True
When you said you “created an association which works“ how exactly did you do that? Can you post a screenshot?
Like I said, I’m not up-to-date, but there used to be multiple different ways in ST of setting up an auxiliary switch as the trigger and a master switch as the target, so the details are important in troubleshooting.
I decided to go ahead and post this even though other people will probably have to help you with the next steps because we have previously had people in the community who didn’t realize that the association group made a big difference and that it might be different from model to model. So they were trying to add the 9640 to association group one for the 9642, and that’s not going to work with these models.
Thank you for your response. First, you are correct. It’s 9640 and 9642. Also, I meant automation on Smartthings, not association.
The dimmer switch in the room works but the companion switch does not.
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Hmmm… I don’t know for sure, hopefully some other folks will chime in.
Meanwhile, what happens if you create an automation which just turns on the master When the auxiliary turns on? No toggle. Does that work?