item contains more than one enclosure
The question as to whether an RSS 2.0 item can have at most one or any number of child enclosure elements has never been completely resolved.
The current state of RSS enclosures seems to be: there are some tools that produce multiple enclosures per item. And there are some tools which can't handle multiple enclosures per item. Both set of tools can point to the spec to prove that they are "right".
Here are a few links to prior discussions on the topic:
Join the rss-public mailing list and contribute to the discussion. Meanwhile, be aware that multiple-enclosure support varies widely from tool tool to tool, and therefore should be avoided if at all possible.