They're not. While the data spaces may not overlap, the file table very likely will (as much of it will fit in a single encryption block), so any parallel changes collide there in even the best circumstance.

I program out of my TrueCrypt volume while in Windows, unmount it, and let it sync. It's a super-easy backup, and DropBox is the only one I've used that handles a 1GB file correctly, and it does it without a hitch.

Worst-case-scenario is you do cause a collision, and you just mount both volumes at once and copy stuff to the one you want to keep.