query with multiple custom fields

I am trying to use a wp_query with multiple custom field key/value pairs. From the results I get it seems that only one key/value pair is actually being used in the query. for example, with the argument:
meta_key=post_group_head&meta_value=no&meta_key=post_group&meta_value=test
I would have expected to only retrieve posts with post_group_head=no AND post_group=test.
However, what I actually receive is posts with post_group=test and no filtering based on post_group_head.

Is this expected? is there a way around it?
Thanks,
joe