Sharing username/password between two databases

Good evening!

I’m working for a company that is making a fairly large site using MU and a custom shopping cart (built internally). The problem is, the cart uses it’s own database (it’s pretty intense).

Here is what we need to figure out.

We want to share the username/password between the shopping area and the community. Either a way to bridge the two fields in the database or a way to nix one (WPMU) so that it pulls data JUST from the cart database. If someone changes their username/password in one database we need it to go across the board for both the ecommerce and the community.

We’re launching our ecommerce site this month and I want to build the community using WPMU. This is the only holdup on that.

Does this make any sense? Does anyone have any thoughts on this? We obviously don’t want to use OpenID for a shopping cart so that’s out of the question.