Here's what I did to work around that, first of all I've used an info field in iTunes (for instance, you could use "Grouping" or any other field) to roughly Group my Music into different Genres, so I have Rock, Hip-Hop, Electronica, Pop, etc. etc. .
Next, I created 3 smart Playlists, calling them "internal storage", "sd card 1" and "sd card 2". I then did some math and combined the Music Genres in a way that I get to like 160GB or so for the SD Cards, so I have for instance all my Metal, Podcasts and Audiobooks in the "SD Card 1" Smart Playlist. All I did for this was go to the "SD Card 1" smart playlist and choose "Grouping is (Metal, Podcast, Audiobook)".
This method allows you to see the size of Music Groups combined together in that smart playlist when you view the smart playlist (status bar at the bottom), so you can find out how to combine the Music Groupings to fit them on your SD Cards and the internal storage.
If you now have 1 Smart Playlist "SD Card 1" with 160GB music in it, 1 Smart Playlist "SD Card 2" with for instance 150 GB in it, and so on, you can connect your DP-X1, go to the Taskbar, right click the iSyncr symbol and start selecting the different storage options and just sync the corresponding smart playlist to it.
I hope this helps and that I didnt explain it too complicated haha.