I am not using any template. Simply 8.5 X 11 single pages.
I believe that means you would select "Blank Page Sizes", under Publication Types, and then select the 8.5 x 11 size if you were starting a new document. Anyway, I believe the solution may be to create a Style or modify an existing Style to set line spacing at 2 lines. Then, by applying that Style to your document, I believe the selected spacing will be applied to the whole document. Styles can be saved and applied to new documents.
That's really all I can suggest. Since I really haven't used Publisher much, I don't think I can be of more assistance. But, Styles appears to be work the same in Publisher as in Word, and I believe using Styles may be a solution for you.