Why manage multiple credential databases for each Odoo website when one can help?
If you want to accomplish the same then just go for the Odoo Website Single Sign-on Module. Single Sign-on in Odoo Website facilitates your customers to access multiple Odoo websites with single login credentials.
Are you looking for an appropriate solution to let your customers log in to all your websites with credentials of one primary website? If yes then here, you have got what you were looking for.