I have not looked to deep, but only found this:

src/pytest_cov/engine.py
108: stream.write('Coverage XML written to file %s\n' % self.cov.config.xml_output)

Therefore it appears that the output path can only be configured through the .coveragerc file.

It would be nice if this could be passed in through an option to pytest-cov, e.g. --cov-xml-output=coverage2.xml.