I got two Zen Cart shops installed on a decicated server.

One is for North America the other for Europe. Clients are supposed to order US products from the US shop and European products from the
EU shop. This works fine with the SEO Module.

In the test environment I have created one shop with all products, delivery and payment options. I have deactived the EU products in the US shop and vice-versa.

A separate .htaccess file in each shop directs the client to the other shop and one could only spot from the Url in the browser in which shop you are in the moment.

So far, so good, but I sometimes run into the issue, that for example a client already registered in the EU shop wants to purchase something from the US shop. They then try to login with their username and password and this has to fail obviously, because they are not registered in that shop.

Would somebody know, if it is possible to use one database for both shops, so if somebody registers say in the US shop he could use the login details in the EU shop, too? Another advantage I see is that if they update for example their address, it would happen in both shops.

I am just a bit afraid if changing to a single database would cause performance issues and I am also unsure if it works with all the modules installed.

For the US I got Google Checkout for card payments and wire transfer and no sales tax (shop is Delaware based), but in Europe it uses Moneybookers for Card payments and IBAN Transfers. I also installed the VAT for European Companies Module there.

Has anyone already tried such a setup and is it possible?