One customer login for multiple zen carts sites

After thinking on this some, I'm willing to say it can't be done effectively.

The client will have multiple zen carts sites on his server but he wants a visitor to sign up only once and have that account pushed into all the other carts, ie, other databases. Essentially synching the customer data (for login purposes only so that the visitor doesn't have to create accounts for each site).

The reason he wants this is so that each cart (each different restaurant) can take differing methods of payment.

Anybody with strong php / datasbase skills want to chime in on this?

If the sites used the same db with separate table prefixes, it would probably be practical to code lookups/storage in other sites' customer tables. It could obviously get dangerously messy if not done right. Separate db's would be trickier, and may require significant custom coding if practical at all.

I hadn't thought of putting them all in the same database, but of course that's possible. Thanks for that bit of more progressive thinking!