The author of this plugin has indicated that they are not longer fielding questions and providing support for this plugin. The plugin is provided as-is.
You may still post a question, but any responses will come from members of the community, not the author of the plugin.
Would it be possible to make a minor edit so you can assign the second avatar to entire groups instead of individual members?
Unfortunately not, as there is no way to see what group a member is in apart from the current member viewing the page. So for example, this topic you are in, there is no way for you to know what the group ID is for myself.
Actually it is possible, I hadn't noticed that the ID is included in the user link now. I will look into it.
Only possible for mini profiles it seems. It wouldn't be possible for the profile page.