The easiest way to achieve this currently is one blog with multiple topics.
Multiple topics can be listed separately - in fact, topic-specific pages are created automatically, but the digest (and the main blog listing page) will contain posts from both topics.
For adding a filter like this, you have to use some scripting to display the posts with the active topic and hide the remaining. If you have any queries re this, please let me know.
The easiest way to achieve this currently is one blog with multiple topics.
Multiple topics can be listed separately - in fact, topic-specific pages are created automatically, but the digest (and the main blog listing page) will contain posts from both topics.