To download the data from the detail sheet you need to select the detail sheet and go to download crosstab, As per my knowledge the data get download with crosstab , based on the selected sheet.
By default in Tableau, whichever the worksheet you place first in a dashboard it downloads the file when you click on crosstab download option. If you want to download sheet2 or any other sheets, click on top of that worksheet and then click on crosstab download option.
As per your ask, you cant download both the worksheets data together in download. however you can download full data or specific sheet data by clicking on the Data download option.
Hope this helps!
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