Union multiple BigTables and query them

We have a product where the data of our customers saved in GCP BigTable instance. Each customer has it's own table for it's own data, but all the tables located in the same instance of BigTable.

Now our new requirement, is to have a Superuser that able to see the data from all customers and able to query it in kind of "union" way...query all tables at once. So I'm trying to find a way how to solve it, without changing the way how customers data is saved and without duplicating the data.
The amount of data that is stored for each customer is about 6TB, and there might be 10-20 customers on the same instance of BigTable.

Thank you!

As the read requests for Bigtable use the stream to get back the contents of the requested rows.This use would be multiple rows can and reads ,it is recommended that you make use of Client libraries instead of direct API calls,which use the ADC for authentication.So the viewer roles set for your user on the tables should allow you to read data.

You may try and use the SQL legacy keyword UNION ALL to combine the select results from multiple tables and fetch the data as you need with the row keys and use the joins to avoid the duplication of data based on the table columns and structures you have currently set up in the schema.Please find the below example

SELECT * FROM `table1`
 UNION ALL 
SELECT * FROM `table2` 

It is to be noted that the reads from the tables of the size you currently have would have an impact on the query performance and would also not be feasible at a long term.Instead you can create views and make the latest reads or check the data there check this official documentation for optimized use of database service for cloud big table.