Is it possible to share customer details between two shops?

I have a shared hosting account. I have a zencart installation in one folder ("mydomain.com/first_shop") and now want to have another installation in second folder ("mydomain.com/second_shop"). I want customers that are already registered in first shop to access the second one with their email and pass from first shop. Also, all new customers should be able to access both shops with registration details from either one...

The two stores will use different templates and sell different products. All I want to share between the two stores is customer details, nothing else is important. Basically, this would mean that a customer who has registered in one shop doesn't have to write name, address, phone number etc. in the other shop when ordering from the other one.

How can this be done?

P.S. I'd like to set it up on localhost xampp before going live...