Since it was bumped: Altoholic will do this, however you have to import the data from one account to another manually and it will will only update data between accounts when you do it. As MiRai put it, not without tedium. I used to do this, but have since stopped. It's just as easy to open all 5 Altoholic windows and add them together when you really need to know.

I do recommend Altoholic, though. I use it mostly for searching for things, and it's search window will search inventories, crafting patterns, and such. You can view a whole account from this window with it tracking everything from quests, professions, gold, inventories (bags, bank, and guild banks), rep... I can't think of anything you can't track.