I could see a couple of options to accomplish this:

1. You could use Yahoo Pipes to quickly and easily generate a new feed with the combination of the 2 feeds you have already found. No code to write, easy to get started.
2. You could write the code to combine the feeds manually as you suggested. Depending on how you plan to display or use the combined feed, you will want to take care in sorting to interleave the 2 feeds based on dates.
3. You could use the YouTube Data API to request the activity feed for the user. This isn't as easy to try in a browser or RSS reader because you need to have, use, and send a developer key as part of the HTTP request headers in order to get results. Then you will want to filter out activity feed items that are not of interest to you - e.g. when a user subscribes to a channel, comments on a video, etc. According to your question you will just want to grab any entry elements where the category term is marked with 'video_uploaded' or 'video_favorited'. Here is the link to the documentation on using the YouTube activity feeds through the YouTube data APIs: Documentation on YouTube activity feeds

If you own the videos, or they are licensed under a creative commons license on YouTube then you will be able to combine them with the editor. Follow the steps below to do so:

1. Navigate to the video editor Video Manager -> Video Editor

2. Now that you are in the editor select two videos (or more) and drag them into the timeline one at a time.

3. Proceed to name your video and hit publish

Now all you have to do is wait for the videos to process and you should be good to go with a merged video.