Using data from both list and a document library in sharepoint data view web part

You could use a connector which will simplify your development process a lot, in example http://www.bendsoft.com/net-sharepoint-connector/.

With such a component you simply connect to your lists as if they where ordinary SQL tables and select the data you want from each list and output it any way you like.

In example

string query = "SELECT ID, LinkTitle AS Title FROM list";
conn = new SharePointConnection(connectionString);
SharePointDataAdapter adapter = new SharePointDataAdapter(query, conn);

DataTable dt = new DataTable();
adapter.Fill(dt);

Or using a helper method to fill a DataGrid

string query = "Select * from mylist.viewname";
DataGrid dataGrid = new DataGrid();
dataGrid.DataSource = Camelot.SharePointConnector.Data.Helper.ExecuteDataTable(query, connectionString);
dataGrid.DataBind();
Controls.Add(dataGrid);

You can see some more examples here, access list in sharepoint 2007 using c#

How you wish to bake up the data is up to your imagination, but it will simplify your workflow and SharePoint development for sure :)

If you are looking for a way to put two lists in a single site in a SharePoint data view web part, then Mads Hansen answered your question. However, if you are looking for roll up lists from different sites under same site collection, you may need tools like SharePoint List Collection or go to codeplex to find some web parts. And if you attempting to combine list data from different site collections, which I hope you are not, you need to get your hand dirty and also you may have to compromise data security, at least I did not make that happen:-)