I havn't tried the AirSync part but for regular USB syncing I was able to get it to use ext_sd by deleteing everything DT had put on the internal SD, shutting down DT and iTunes, connecting phone then under Storage Management removing the drive letter from the internal SD card. (obviously this is a Windows only solution though ejecting the internal one on a Mac befroe firing up DT might work too).
Once you remove that drive letter restart DT and it showed and synced right to the external SD.
I went this route mainly because I only want the internal SD used by apps, pics, etc. I do believe this will break the syncing of pics and videos thru DT, but not actually tested.