I can not think of any way to achieve this other than having two enum attributes: enum1 - (A,B,C) and enum2 - (A,B). You can make sure their values match by using a Before Commit microflow.
Perhaps, some else has a better idea.
The latest version of my GridSearch widget package might meet your needs. It contains a searching widget where you can create your own XPath filters. You could use this with conditional visibility to display either a widget with 2 options or a widget with 3.
Download here (latest not published in the app store yet):
Variable Enum values are not possible in Mendix, I would fix this using an entity and add a reference set from the User group to select the possible options. But as far as I understand the search bar is fixed and will show all possible options. So to make make this possible you should have to add the data grid twice and add a visibility condition.
Try using this widget: https://appstore.home.mendix.com/link/app/56183/Finaps/EnumSelector
I think that will solve your problem.