It looks like difficult but from my point of view for this solution, you can use User filter in Data Source.
Please let me explain for this.
1.) Create one column in each data source and put value name of the manager like if you have sales manager "Alex" then put "Alex" in all row of new created column and same process with another data source.
2.) Now merge(Union) both data source together and create one table.
3.) now create dashboard and use "User filter" or "Data source Filter" for that column where we have kept name of "manager" where you can restrict who can see which data.
I think this can solve your problem.
Please mark helpful if you are able to solve your problem with my solution.
Please let me know if you don't understand my concept. I'll try to explain in different way.
It will be extremely helpful if you could please create an example workbook for me with dummy data. I would really appreciate that.