Other issues include:
What if 1 hopper has space and the other has a hard drive full? Customers would get upset if they started to lose programs with free space.
If some episodes of a show go one one drive then the other? EHD transfer issues?
You lose one of the 2 hoppers?
What if one loses power but not the other. Then it is plugged back in, how do they figure out how to recover. (i.e. the one that was the master got unplugged, the slave decided it was the master then the master came back, which one is in charge)?
The list of issues can go on and on. The "best" solution would probably be a 6 tuner Hopper, of course you have a single point of failure, but much easier time of dealing with it.