In general, you can't, and don't want, different directory servers telling you about different nodes. Imagine what would happen if one directory server was compromised, and told you about a set of attacker nodes.
Now, one place where this setup becomes interesting is if you have a separate network -- either the corporate internet, or some variation of IP V8 (maybe a very large ISP, like China, decides to use their own network numbering with a translating boarder). Then, you could have two sets of directory authorities, for two sets of destination IP ranges -- one set of directory authorities for these IP's, and a different one for those IP's. Then, you might wind up building two tunnels, one to get to the gateway, and a second to traverse the second network on the other side.
