RQM API: Get executionworkitem using as filters testplan and testcase

I am working with RQM v4.0.4 and 4.0.6, and cannot migrate to upper versions.
I am trying to get executionworkitem related to a specific testplan and testcase. This because a testcase can be linked to more than one testplan.
I am using this filter schema:
<feedUrl>?fields=feed/entry/content/testplan[title='test plan title' or description!='test plan description']/*

taken from the RQMApi reference.
the filter i am using is:

<feedUrl> ?fields=feed/entry/content/executionworkitem/[testplan[@href='testplan_href'] and testcase[@href='testcase_href']/*

I think I can do this, because, from version 4.0 on, executionworkitem can be filtered with both testplan and testcase, according to the same reference:

• /testcase/@href
• /testphase/@href⁶
• /testplan/@href⁶

What I get is a list of executionworkitems only related to the  testplan, as though the "and" operator was not in effect, w/o any property.
Would you please be so kind to tell me where am I wrong?
I really thank you in advance for any hint.
Ciao
Sandro