You can create a new table, just with two rows that say True and False. Link this to all of your datasource tables and then use it in the slicer.
Another way would be to combine your csv files together. If they all have the same columns that could be worth doing anyway. You'd have another column saying which file any row originally came from that you could use to separate them.
You can create separate dimension tables to use as a common filter for all your tables. For instance, you can create a table with a single column, that has two values, True or False Then create a relationship between this table, and each of your data tables on the True/False column. On the report view, use the true/false column from the dimension table as a slicer. Once you select a value from the slicer, your data tables will be filtered accordingly.
Please mark this answer as the solution if it resolves your issue. Appreciate your kudos! 😊