I'm pretty sure that if one board "hangs", the other board will notice a fault in the inter-board communication, and begin the default safety procedure.
And, the odds for a specialized non-modifiable device to hang is considerably lower than that of a generic computation device, with it's runtime being modified all the time...
The hardware will always respond the same if it's functioning, and the software will always give the same results to the same data, therefore it's easier to keep things running stable than with interchangeable hardware, with modifiable software...
Therefore, it's safe to assume for one computer, that if the other computer isn't responding, something has gone terribly wrong...
And, remember, the redundancy in the Segway isn't to make sure you can keep gliding, but to make sure you don't fall... If one level of redundancy kicks in, that means you not longer have any other redundancy at that level, so next fault will be fatal, so the safest thing is to shutdown, which is what the Segway does.
You can't quite compare it's redundancy with that of a computer...
Bob: I'm pretty sure that is just how it runs, no "Turtle" mode there
jrwilcox: I have yet to see any specs as well...