Is it possible to share a packaged workbook demonstrating the issue? Just a small amount of data and sample viz would probably be enough to get across the issue and desired results. Please also let us know what version of Tableau Desktop you are working in so as to prevent users such as myself from assuming it's the latest version, 10.1.
Here's a .twbx made in 10.1 for you. In the first worksheet I've done a bubble chart sized and sorted according to the same data source, and the second worksheet brings in a secondary data source. In this example, I'd like to size the bubbles based on data source #1, and sort them based on data source #2. For the actual build I don't have the ability to create an extract of a combined data source, so this would be preferable.
Bubble Sort.twbx 1.7 MB