One slicer for two tables

Here's what you generally do. Just copy and paste your query for one of your tables. Remove all of the other columns other than the one that you are interested in, let's say "Year". Remove duplicates. Save as your Year table. Now, if a new year is added in the data, you will automatically get it in your Year table.

If you have some values in one table and some in another, then what you want to do is do as above and then repeat for your other table and then create an Append query to append them together and remove duplicate values. Now you have a dimension table that has all of the possible values in both of your tables.