Currently, AppSheet does a lot of things automatically when it detects that two tables have a relationship. But if you want to maintain bi-directional, many-to-many relationships, you need to manually define a join table and do some other work to manage the synchronization of the join table. In Zoho Creator, Airtable and other products, the join table is created and managed automatically and easily configured within a single field definition.
In AppSheet, the interface for enabling this could be quite easy to use. When defining an ref-based enum or enumlist field, the user would simply indicate whether or not the relationship should be bi-directional.
Note: I updated this today in the hopes of making it easier to understand.
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