Two separate feeds. Two major reasons for this.
1. RSS complications having 2 or more media files in one post, see top of this page: http://wordpress.org/extend/plugins/powerpress/faq/
2. If you do the 2 separate blog posts route, one video and one audio, you definitely need separate feeds, otherwise the last blog post (e.g. audio) will be the "latest" and what is downloaded by the subscriber. The 2nd latest podcast episodes (e.g. video) will be skipped, since they were both released at he same time and the last one (audio) is seen first in the feed.
The PowerPress solution to this problem is called "Podcast Channels", which allows you to write one blog post and maintain 2+ separate podcast feeds. When you enable the advanced option then go to the custom channels page, the examples documented will assist you with setting up the channel best for your situation.