Here's what I got:

User database

Product database

In the User db, I have a product portal in the main layout, that pulls all products sold to that user. However, in the products db, I have Field XYZ which contains multiple values.

I would like to create a second layout in the user database that only pulls portal records for which Field XYZ matches a particular field in User database. I'm guessing this involves setting up some additional or alternative relationship but I'm not sure what that relationship should be.

Is it possible to define a relationship to match both the User ID *and* the second XYZ field and then use that relationship for the second portal?

Create a calc field in User that combines the User ID and the actual value in XYZ that you want to see (i.e. UserID & "Value 2").

Create a calc field in Products that combines User ID and XYZ (i.e. UserID & XYZ).

Now create a relationship in User between these calc fields. You'll probably want to disallow creation of related records for this relationship if it's a view-only situation.

Cool, thanks!