“Q: I have two different blogs but want to offer one consolidated feed. Does FeedBurner allow combining of multiple feeds into one ?
A: No, FeedBurner won’t merge your multiple feeds into one but you can use external services like Yahoo! Pipes, SuperGlu or RSS Mixer to create a single feed from your various blogs and then syndicate the merged feed via FeedBurner. Watch this tutorial on using Yahoo! Pipes to merge multiple RSS feeds into one FeedBurner feed.”
From FeedBurner FAQ – relevant Google search was ‘rss feedburner multiple sites in one feed’. Since you do not specify if you have tried any of those third party things, and if so, where exactly you got stuck, it is not easy to give more specific help.