Though it’s not explicitly specified in the spec (as not much really is) I thought I’d mention here for future reference that RSS 2.0 really only expects there to be one enclosure per item.
This question comes up from time to time, and I’ve resisted answering it directly, thinking that anyone who really read the spec would come to the conclusion that RSS allows zero or one enclosures per item, and no more. The same is true for all other sub-elements of item, except category, where multiple elements are explicitly allowed. The spec refers to “the enclosure” in the singular. Regardless, some people persist in thinking that you may have more than one enclosure per item. Dave Winer