We have built such a customization in the past where two stores of differing base language shared products and inventory of the same SKU number (that is the key).  What our modification did was synchronized the inventory, like you mentioned, with one of the store's being the "master store".  Whenever a product was ordered in either store, the inventory levels in both stores would be adjusted.  We built a mod for the admin panel that allowed for manual inventory adjustments which were updated to both stores.

 

At the same time we were working on this project, my development partner wrote an additional modification that would allow for product and pricing updates from the master store ... we could not use this piece on this project mainly because the customer's stores were in two different languages and two different currencies.

 

Now I do suspect the method we use would have some limitations if we were to branch it our to multiple stores as keeping things in synch could prove challenging (Synching 2n+ number of databases is a lot different than synching just two databases).  Neither I nor my Development Partner are versed in XML but I do think this would be the way to go if you are wanting to expand to more stores.  I would certainly be willing to either pull in a XML developer (or work with one from the community) to collaborate on finding a solution.