I wonder if it has to do with the fact that the other Mac is already reserving the disk for use with Time Machine.
I have a very similar setup and could try this myself on monday when I'm back at that computer.
Until then, I suggest you try adding another small partition to the DS, e.g. by using any free space left on the internal disk, or by pluggin a USB drive into it. Then make this volume available via AFP as well, and see if both your Macs can access it normally. That would prove that it has to do with the particular shared partition and not with the network setup.