Mike is right, but there is a way to have multiple media in one post, it just causes more problems than what it was intended for. If you did have 2+ media files in 1 post, iTunes will pick up the first one, and other feed readers/podcatchers will pick up the last one (They are only looking for one media file). The standard was intended for 1, just like there is supposed to be only one title, description and guid for every post in the feed. You can do more (PowerPress will not let you, but if you wrote your own RSS feed from scratch in a text editor you could), but as you can see, it will drive your audience bonkers.