Unfortunately, I haven't really seen a valid system for such trees. Not many people plan to have a leaf coming off of more than one branch, but I do understand how this could be helpful. Therefore, I'd leave you with four possible solutions:

• Build your own tree module that will do what you need it to do. (One idea I've always loved, and use in my own home-spun tree algorithms is the idea of a sort key, which can tell all the parents of a single id, and could possible allow for two parents. A sort key looks like (root_id,branch_id,parent_id,leaf_id), assuming one has a root, a branch a sub-branch, and a leaf. You could then hack into that allow for (root_id,branch_id,[parent_id|other_parent_id],leaf_id), and parse into that with a split, and a regex. This would be helpful if you want to get the history of a leaf, but developing this into a full-scale tree module would take some time.
• Duplicate the leafs, and use DBIx::Tree (This means that you'd leave all the same information, except for the id, and parent id.) Unfortunately, it also means that you'll have to duplicate children of that parent.
• Develop your own tree algorithm, or edit DBIx::Tree to allow for two sets of parents. (This actually wouldn't be too hard.) I would suggest this solution, or the next one. My advice would be to allow DBIx::Tree to search for an or clause in your parent_id, and then have that clause build to both parents. Unfortunately you'd have to change the algorithm a little further to allow for sub-leaves to be built in both places. I'll leave that as an exercise for your mind :)
• Look some more for a cpan solution, because I don't know it :)

Gyan Kapur
gyan.kapur@rhhllp.com