Hi-
Someone recently asked a question about this post, and I wanted to clarify that there is in fact a workaround to searching on custom fields from two different assets at the same time. Here is an example that finds Stories and Defects. Stories have a custom field called Custom_MyField2, and Defects have a custom field called Custom_MyField. The value 'Find Me' is contained in one Story and one Defect in these fields. This query will find both (***tested on VersionOne 8.3***):
http://servername/v1instance/rest-1.v1/Data/PrimaryWorkitem?where=Defect.Custom_MyField='Find%20Me'|Story.Custom_MyField2='Find%20Me'&sel=Name,Defect.Custom_MyField,Story.Custom_MyField2
The key here is qualifying the type on the attributes you are searching for. That is why both custom field names are prefixed with either Story. or Defect., and notice that I use an 'or' vertical pipe | in the where clause.
The APIClients may not build the search strings this way - I haven't tested that. But at the least you should be able to get the raw results back as xml by sending this url to the V1APIConnector object (again, this only applies if you are trying to do this query using the API/SDK).
-Maggie