As an addition to anyone else who has this type of issue:
To get one report from two groups of tests run from the same assembly, define two Windows Batch Command build steps, each with its own category and result file :
"%nunit%\nunit-console.exe" "%WORKSPACE%\MyTests.sln" /include:TestCategory1 /xml=nunit-result1.xml
and
"%nunit%\nunit-console.exe" "%WORKSPACE%\MyTests.sln" /include:TestCategory2 /xml=nunit-result2.xml
Add a Post Build action to Publish the NUnit Test result report and define the XML filename as *.xml.
The Nunit plugin will concatenate the different test results into one result that can be viewed and emailed.
This way will also solve the OP's question.