You can join few RSS feeds very easily by using rss mix. Simply copy-paste rss urls you want to join and click Create. You will be moved to your new RSS which you can subscribe.

RssMix unfortunately has problems with polish encoding. After search I have found Yahoo! Pipes. It works perfect is very easy and has more functionality which can be used in future. Yahoo! Pipes require logging in but except Yahoo! account you can also log in using Google and Facebook accounts.

After logging in click Create Pipe. To add RSS feeds drag and drop Fetch Feed block from Sources Panel on the left. To one Fetch Feed you can add only 10 RSS feeds but you can add few Fetch Feeds and union them together. To be sure Yahoo! joins that pipes in correct order I added Sort operation and sort joined pipes descending by publish date.


You can check output of your pipe clicking on Pipe Out block. There's even a debugger!
When your pipe is done save it and click Run Pipe on the top of the page. Yahoo will take you to your Pipe Page. There should be RSS button which will take you to your new RSS page.