Issue
Customer can't use the <xref> element for topics that are in a different ditamap
Solution
Correct, this is something that isn't currently supported. The workaround for this would be to take the different ditamap files and nest them under a parent ditamap so that these would all be considered one ditamap and able to be processed by ePublisher. For example, your parent ditamap would look like this:
<topicref format="ditamap" href="map1.ditamap" navtitle="Map 1"> </topicref> <topicref format="ditamap" href="map2.ditamap" navtitle="Map 2"> </topicref> <topicref format="ditamap" href="map2.ditamap" navtitle="Map 3"> </topicref>