This tool takes a list of feeds and interleaves them to create a new combined feed.
RSSMerge is 'bring-your-own-state'. This means you need to paste your feed URLs into a gist or something, like this one.
The format needs to be one feed URL per line, like this:
http://site.com/rss https://coolblog.co.uk/blog/atom.xml http://newspaper.org/feed.rss https://blog.company.com/feed.atom
Once you've created your Gist/Pastebin/plaintext-thing, paste the the raw content url here:
Made in a little but of a hurry by charlieegan3. Code on GitHub.