I don't think that's possible--the hardware keeps them separate.
My workaround is to put all the music I know I'm going to want with me all the time in the Sansa's internal memory, and use the MicroSD for my constantly rotating music, audiobooks, etc. I don't sync anything, but if I did, I'd just set up the Micro SD card as the sync device.
e280 + 16GB MicroSDHC = Mondo sweetness.