The missed detail about splitting the tests in several files was the definition
of the webtest taskdef:
<taskdef resource="webtestTaskdefs.properties" />
Including this in the test files I can use an ant file to link to all the tests:
<project name="Canoo Webtest Sample" basedir="." default="all">
<target name="all">
<ant antfile="pageEmbedderWebTest.xml" />
<ant antfile="pdfWatermarkWebTest.xml" />
</target>
</project>
And the test:
<?xml version="1.0"?>
<!DOCTYPE project SYSTEM "../resources/dtd/Project.dtd">
<project default="test">
<taskdef resource="webtestTaskdefs.properties" />
....
Now it works...
And I can have 1 webtest per page in the project....