DomainServices are all about providing Domain specific data for your application (by "Domain" they mean the area of business, rather than TLDs, which can lead to some confusion).
If your data was coming from two external services you would create a new data structure to hold the combined data and serve that to your application. That is no different to your situation of two databases so...
Your Domain service should be serving up the data your application wants/needs and no more. Create a class that has all fields you want/need and use LINQ to populate them from the two databases.
RIA is all about providing an easy way to map C.R.U.D. calls to methods. What you do in those methods is up to you, but the focus should be on the needs of the application and not what happens to be in the tables.