XPATH or Join with XQuery

I have two Tamino database - one stores customer data which is shared by many applications and the other customer activities. When I use Tamino Interactive to query against the customer activity data, sometimes I may need to gain access to the customer profile. In this case, is it better that I use x-path to map portion of my customer data on my customer activities document or CAN I use Tamino interactive to query against both the customer document and the customer activities document simultaneously? If later, how can I do that?

Please advise.

Sorry, all references in my earlier notes should refer to X-Node instead of X-Path.