SSRS set datasource of report model
Depending on the version of Report Builder, you can only build from a single data source.  I'm pretty sure this is still the case with SQL Server 2008R2, although Report Builder 3 can consume data directly from a database without needing a report model, although you need to build data sets using the individual connections and I don't think you can group items from different data sets by a common value.

You can do this gouping with SSRS (see this article for an overview), although I can't see an obvious way to do it in RB3.  This article indicates that you can't nest items based on different data sets, so it's looking like common rollups aren't possible in RB3.