item contains more than one
According to the RSS Advisory Board's Best Practices Profile, support for the enclosure element in RSS software varies significantly because of disagreement over whether the specification permits more than one enclosure per item. Although the author intended to permit no more than one enclosure in each item, this limit is not explicit in the specification.
For best support in the widest number of aggregators, an item should not contain more than one enclosure.
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.
Not clear? Disagree?
You might be able to find help in one of these fine resources.