Append queries refreshing separate tables and then append query re-querying source also?

Hi,

I am using an append query to union a snowflake data source with a SQL server data source, the process is working but I have a query regarding the refresh process, it seems currently that I bring the two tables in to Power BI so I can append them, but then the append query then goes back to the source and takes the data from the source again, whereas I would have expected that process to take the data it has just pulled in to the power BI environment and then simply union the two together internally. The data sources combined are circa 20 million rows so ideally for speed and size I would rather just have the grab done once and stored once.

Is there any option to achieve this without looking at datamart etc (we are using premium so is an option)?

Any advice much appreciated.

Thanks