Right now if you try to import multiple $othersite journals into a single DW journal, it will pick up all entries from one account and some-or-none of the entries from the other account. (This is a bug, and we're working on fixing it!)

You can import your journal multiple times, though -- I re-import my personal journal once a week or so. So if you import, then make another post to your $othersite journal or get more comments to your $othersite journal, you can re-import it and just the new stuff will come over.