First rule of home automation: “the model number matters.“ Also, reading the directions can help.
From the instructions for the Inovelli association tool:
The tool requires special code to be present in the device handler so it will only work with Inovelli devices.
So, yeah, that’s not going to work.
Tweaker should work. This is a DTH (Device type Handler), not a smartapp.
If you don’t know the difference, start by reading the following community FAQ. Although it was written for the 2015 version of the app and the publishing instructions will be out of date, the terminology and concepts Still apply. So make sure you understand those before going forward, it will make everything a lot easier for you.
[OBSOLETE] FAQ: An Overview of Using Custom Code in SmartThings (SmartThings Classic, Groovy Code)
Once you’ve read that and understand the concepts, you are ready to use the tweaker. What you are going to do is temporarily assign the tweaker as the DTH for the accessory switch. Then you should be able to go into the smartthings app and set the parameters the way you want, including the association groups.
Once you get those set, they will save into the switch itself. So then you go back to using the original DTH for every day use.
The switch will not work while it is assigned to the tweaker: you are only going to use that to set the parameters.
You don’t need to do anything with the master switch, it will be fine the way it is.
Here’s the thread with the Tweaker code. Read the first post so you understand what it is, then jump all the way down to post 209 in that thread. It has a version which has been updated for the 2018 version of the smartthings app, and I believe it also works with the 2021 version.
If you run into any problems with the tweaker, just ask in that thread and someone will help you. 
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(Make sure you make a note of the original DTH that the accessory switch is using so you can go back to it after you’re done with the Tweaker.)
Good luck! I really like the Eaton devices, they are well engineered. They should work well for you once you get the groups set up correctly.
Oh, and before going further, what’s the exact model number of the devices? It can make a difference.
Also, assuming your accessory switch is RF 9642 (again, the model number matters), you will need to use Association group 3 in order to get dimming on the master switch. Don’t use Association group 1, and association group 2 is for on/off switches, not dimming.
https://products.z-wavealliance.org/products/3157/assoc?noFilename=True