Note for later:
Bug in biztalk mapper with multiple messages.
BizTalk mapper does not support multiple input messages where 2 (or more) use the same namespace. I get a compile error on the btm.cs file -- a file I cannot even edit.
Workaround: Use two maps. In second one, take the result of the first one, mass copy, plus add the second message.
And a *somewhat* related bug report: http://support.microsoft.com/kb/883376