DMG 3.5, page 215, Magic Item Descriptions, Prerequisites: "If two or more characters cooperate to createe an item, they must agree among themselves who will be considered the creator for the purpose of determinations where the creator's level must be known. (It's generally sensible, although not required, for the highest level character involved to be considered the creator.) The character designated as the creator payes the XP required to make the item."
I can't find anything about sharing the XP cost to create an item.