I'm booting from the internal, MMC2, and have virtual memory turned on.
I plug in the USB cable, and my N810 pops up a messsage that says:
Unable to connect via USB.
Memory card in use: internal memory card

I go to my PC, and two USB drives are listed.
I can access the first one, read, write, all good.
Accessing the second one gets me a message that says:
There is no disk in drive G.
Insert a disk and then try again.

Is that not what y'all are seeing?

This is, of course, if the N810 is on when I plug it in.
If I plug in the USB before booting the N810,
then I have full access to both cards and virtual memory is disabled.
I don't think you're going to beat that case,
because the drives are shared before the OS gets to them.

Bundyo, how have you got your internal card partitioned?