How to import nodes that reference each other using Feeds?
Two importers can work on the same nodes without any problem, but that's not what you need.  Instead, You need to set the importer to update/overwrite existing nodes when they have the same GUID.  Then you run the importer twice on the same xml file.  The first time they won't be populated properly, but because they'll already exist the second time, the second run should fix it.