Feedburner doesn’t give this merging feature which is now given by Yahoo pipes.Combine both of them and get your desired result
Here is a step by step guide given by Amit at Digital Inspiration
Step 1: Go to the Yahoo! Pipes website and Create a new Yahoo! Pipe here.
Step 2: Drag-n-drop the “Fetch Feed” tab from the Sources tab to the main drawing panel.
Step 3: Add all the different Feed URLs in the same Fetch Feed Module (click the plus icon).
Step 4: Once you are done adding all your RSS feeds, drag the lower circle from Feed Fetch module to the Pipe Output at the bottom.
Step 5: Give some name and save this Yahoo! Pipe. Now run the pipe and you will see a new RSS feed – syndicate that feed via FeedBurner so you can even track the feed subscribers. It’s all very simple