What I mean is following:
1. Calibre recipe system is designed to handle only one content structure in a recipe. That means that you can only place one set of rules to extract content for a specific websitticle.
2. Every website has its own unique content presentation structure.
3. So when you mix feeds from more than one sites than you will not be able to cleanup correctly all unneeded stuff from the feed pages since you can support only one wedsite structure per recipe. In short - you can mix the feeds but the result in general will be crap.
4. The only case where you can mix the feeds and get a good result is where feed itself contains complete article text. My example mixes two feeds that exhibit such behavior and that is why it works. Pay attention to the line in recipe
Code:
use_embedded_content = True
That instructs calibre to get the article text from the feed and not to open the URL in the feed.