You can't do math between two data sources if there is no connection between those data sources. You'll have to create a join or blend if you want to do this.
You could do a dashboard where you compare certain metrics side by side in separate vizzes, one for each data source.
You also can't list the tables in a given database in a Tableau viz unless there is some metadata layer you can attach to with a list of those tables. You can see them in the data source tab but you can't reference the table name in a Tableau calculated field. If you have a tool like Alteryx you could write a transform that would expose that metadata to Tableau, but it's not something you can do out of the box. I don't think you can do that with Prep, although I'd be delighted to be wrong about that.