For version 8.0, you can use Bookmarks to do this.   Using a Bookmark to Copy a Sheet to Another Workbook | Tableau Software

In version 8.1, you can simply copy and paste the sheets from one workbook to another.  Copying and Pasting Between Workbooks | Tableau Software

Note that doing this will often result in a duplicate data source, even when combining sheets from two workbooks connected to the same data source--for some reason, Tableau doesn't automatically replace the one you're using or prompt you to ask you if you want to replace it or keep the original, even if it has the same name (in 8.0, a prompt would result--I don't know why they removed this, as it was much easier to understand)..  So, you may have to use the  "Replace Data Source" option after doing this in 8.1 in order to just have the one data source in the workbook.

This is why I created this idea to bring the dialog back: http://community.tableau.com/ideas/2903