This is an easy task. Try going through this link, it has step by step procedure for filtering from multiple data sources
Also, this discussion thread might be helpful
Post a packaged workbook if you still face issues.
The original post has a packaged workbook in it (as a link to a Tableau Public workbook, which can be downloaded from the link provided). Perhaps it was added later, or you may have missed it since it wasn't "attached" to the thread.
To say this is an "easy task" is a little misleading, in my opinion. Also, the KB article you linked to suggests using Action Filters, which I don't think is the appropriate solution for this scenario. The other option is to use a parameter, which can be applied across data sources.
April, I believe this is the approach you'll want to utilize here--this article provides a similar scenario, with a parameter solution I believe you'll be able to easily apply: