To populate a read-only grid with 2 data subsets

It looks like you don't have any relationships configured on your records .Record Relations Maintain relationships , So you can refer the data easily.

If you don't want to maintain relationship create a dictionary with both record data and use it. I prefer the first one.

Why don't you create a rule in Appian or view in DDBB in order to sort and merge the data?

It's definitely easiest to merge the data into a coherent dataset before you try to render it in your user interface. If you're working with Modern (Synced) Records then you can create relationships between the two datasets in Appian and that'll allow you to get at the data as required for each row in your table. If you're still working with datastore entity backed data then create a view in your database that joins your two tables and then you can create a new CDT that is mapped to your view and treat it as one dataset in your user interface.